text: Title: Raise helpful error message when importing optional dependencies that are not installed.
  Body: ### Description
  <!-- Please provide a general introduction to the issue/proposal. -->
  If cartopy is installed without optional dependencies, it would be nice to have a helpful error message when trying to use functions that depend on such libraries. Currently, just an ImportError is raised. It would be more useful to notify the user of the optional dependency (possibly including a documentation link).
  
  <!--
  If you are reporting a bug, attach the *entire* traceback from Python.
  
  If you are proposing an enhancement/new feature, provide links to related articles, reference examples, etc.
  
  If you are asking a question, please ask on StackOverflow and use the cartopy tag. All cartopy
  questions on StackOverflow can be found at https://stackoverflow.com/questions/tagged/cartopy
  -->
  
  #### Code to reproduce
  
  ```python
  from cartopy.crs import epsg
  epsg(32633)
  ```
  
  #### Traceback 
  
  ```python
  ModuleNotFoundError                       Traceback (most recent call last)
  <ipython-input-2-a9ebc24438bc> in <module>
  ----> 1 epsg(32633)
  
  ~/miniconda3/envs/spechomo/lib/python3.8/site-packages/cartopy/crs.py in epsg(code)
     2555     """
     2556     import cartopy._epsg
  -> 2557     return cartopy._epsg._EPSGProjection(code)
  
  ~/miniconda3/envs/spechomo/lib/python3.8/site-packages/cartopy/_epsg.py in __init__(self, code)
       40 class _EPSGProjection(ccrs.Projection):
       41     def __init__(self, code):
  ---> 42         import pyepsg
       43         projection = pyepsg.get(code)
       44         if not (isinstance(projection, pyepsg.ProjectedCRS) or
  
  ModuleNotFoundError: No module named 'pyepsg'
  
  ```
  
  <details>
   <summary>Full environment definition</summary>
  
  <!-- fill in the following information as appropriate -->
  
  ### Operating system
  Linux-64
  
  ### Cartopy version
  cartopy                   0.18.0           py38h88488af_4    conda-forge
  
  ### conda list
  
  ```
  _libgcc_mutex             0.1                 conda_forge    conda-forge
  _openmp_mutex             4.5                       1_gnu    conda-forge
  argon2-cffi               20.1.0           py38h1e0a361_2    conda-forge
  async_generator           1.10                       py_0    conda-forge
  attrs                     20.2.0             pyh9f0ad1d_0    conda-forge
  backcall                  0.2.0              pyh9f0ad1d_0    conda-forge
  backports                 1.0                        py_2    conda-forge
  backports.functools_lru_cache 1.6.1                      py_0    conda-forge
  bleach                    3.2.1              pyh9f0ad1d_0    conda-forge
  blosc                     1.20.1               he1b5a44_0    conda-forge
  boost-cpp                 1.74.0               h9359b55_0    conda-forge
  branca                    0.4.1                      py_0    conda-forge
  brotli                    1.0.9                he1b5a44_2    conda-forge
  brotlipy                  0.7.0           py38h8df0ef7_1001    conda-forge
  brunsli                   0.1                  he1b5a44_0    conda-forge
  bzip2                     1.0.8                h516909a_3    conda-forge
  c-ares                    1.16.1               h516909a_3    conda-forge
  ca-certificates           2020.6.20            hecda079_0    conda-forge
  cairo                     1.16.0            h488836b_1006    conda-forge
  cartopy                   0.18.0           py38h88488af_4    conda-forge
  certifi                   2020.6.20        py38h924ce5b_2    conda-forge
  cffi                      1.14.3           py38h1bdcb99_1    conda-forge
  cfitsio                   3.470                hce51eda_7    conda-forge
  chardet                   3.0.4           py38h924ce5b_1008    conda-forge
  charls                    2.1.0                he1b5a44_2    conda-forge
  click                     7.1.2              pyh9f0ad1d_0    conda-forge
  click-plugins             1.1.1                      py_0    conda-forge
  cligj                     0.6.0              pyh9f0ad1d_0    conda-forge
  cloudpickle               1.6.0                      py_0    conda-forge
  cmocean                   2.0                      pypi_0    pypi
  colorama                  0.4.4              pyh9f0ad1d_0    conda-forge
  cryptography              3.1.1            py38hb23e4d4_1    conda-forge
  curl                      7.71.1               he644dc0_8    conda-forge
  cycler                    0.10.0                     py_2    conda-forge
  cython                    0.29.21                  pypi_0    pypi
  cytoolz                   0.11.0           py38h1e0a361_1    conda-forge
  dask-core                 2.30.0                     py_0    conda-forge
  decorator                 4.4.2                      py_0    conda-forge
  defusedxml                0.6.0                      py_0    conda-forge
  dill                      0.3.2              pyh9f0ad1d_0    conda-forge
  entrypoints               0.3             py38h32f6830_1002    conda-forge
  expat                     2.2.9                he1b5a44_2    conda-forge
  fast-histogram            0.9                      pypi_0    pypi
  fiona                     1.8.17           py38h676c6b2_1    conda-forge
  folium                    0.11.0                     py_0    conda-forge
  fontconfig                2.13.1            h1056068_1002    conda-forge
  freetype                  2.10.3               he06d7ca_0    conda-forge
  freexl                    1.0.5             h516909a_1002    conda-forge
  future                    0.18.2                   pypi_0    pypi
  gdal                      3.1.3            py38h9edfc58_1    conda-forge
  geoarray                  0.10.0           py38h32f6830_0    conda-forge
  geojson                   2.5.0                      py_0    conda-forge
  geopandas                 0.8.1                      py_0    conda-forge
  geos                      3.8.1                he1b5a44_0    conda-forge
  geotiff                   1.6.0                h5d11630_3    conda-forge
  gettext                   0.19.8.1          hf34092f_1003    conda-forge
  giflib                    5.2.1                h516909a_2    conda-forge
  gitdb                     4.0.5                      py_0    conda-forge
  gitpython                 3.1.9                      py_0    conda-forge
  glib                      2.66.1               he1b5a44_1    conda-forge
  hdf4                      4.2.13            hf30be14_1003    conda-forge
  hdf5                      1.10.6          nompi_h54c07f9_1110    conda-forge
  icu                       67.1                 he1b5a44_0    conda-forge
  idna                      2.10               pyh9f0ad1d_0    conda-forge
  imagecodecs               2020.5.30        py38h63741c2_4    conda-forge
  imageio                   2.9.0                      py_0    conda-forge
  importlib-metadata        2.0.0                      py_1    conda-forge
  importlib_metadata        2.0.0                         1    conda-forge
  ipykernel                 5.3.4            py38h1cdfbd6_1    conda-forge
  ipympl                    0.5.8              pyh9f0ad1d_0    conda-forge
  ipython                   7.18.1           py38h1cdfbd6_1    conda-forge
  ipython_genutils          0.2.0                      py_1    conda-forge
  ipywidgets                7.5.1              pyh9f0ad1d_1    conda-forge
  iso8601                   0.1.13                   pypi_0    pypi
  jedi                      0.17.2           py38h32f6830_1    conda-forge
  jinja2                    2.11.2             pyh9f0ad1d_0    conda-forge
  joblib                    0.17.0                     py_0    conda-forge
  jpeg                      9d                   h516909a_0    conda-forge
  json-c                    0.13.1            hbfbb72e_1002    conda-forge
  json5                     0.9.5              pyh9f0ad1d_0    conda-forge
  jsonschema                3.2.0                      py_2    conda-forge
  jupyter_client            6.1.7                      py_0    conda-forge
  jupyter_core              4.6.3            py38h32f6830_2    conda-forge
  jupyterlab                2.2.8                      py_0    conda-forge
  jupyterlab-git            0.22.1                     py_0    conda-forge
  jupyterlab_pygments       0.1.2              pyh9f0ad1d_0    conda-forge
  jupyterlab_server         1.2.0                      py_0    conda-forge
  jxrlib                    1.1                  h516909a_2    conda-forge
  kealib                    1.4.13               h33137a7_1    conda-forge
  kiwisolver                1.2.0            py38hbf85e49_1    conda-forge
  krb5                      1.17.1               hfafb76e_3    conda-forge
  lcms2                     2.11                 hbd6801e_0    conda-forge
  ld_impl_linux-64          2.35                 h769bd43_9    conda-forge
  lerc                      2.2                  he1b5a44_0    conda-forge
  libaec                    1.0.4                he1b5a44_1    conda-forge
  libblas                   3.8.0               17_openblas    conda-forge
  libcblas                  3.8.0               17_openblas    conda-forge
  libcurl                   7.71.1               hcdd3856_8    conda-forge
  libdap4                   3.20.6               h1d1bd15_1    conda-forge
  libedit                   3.1.20191231         he28a2e2_2    conda-forge
  libev                     4.33                 h516909a_1    conda-forge
  libffi                    3.2.1             he1b5a44_1007    conda-forge
  libgcc-ng                 9.3.0               h5dbcf3e_17    conda-forge
  libgdal                   3.1.3                h670eac6_1    conda-forge
  libgfortran-ng            7.5.0               hae1eefd_17    conda-forge
  libgfortran4              7.5.0               hae1eefd_17    conda-forge
  libglib                   2.66.1               h0dae87d_1    conda-forge
  libgomp                   9.3.0               h5dbcf3e_17    conda-forge
  libiconv                  1.16                 h516909a_0    conda-forge
  libkml                    1.3.0             h74f7ee3_1012    conda-forge
  liblapack                 3.8.0               17_openblas    conda-forge
  libnetcdf                 4.7.4           nompi_h84807e1_105    conda-forge
  libnghttp2                1.41.0               h8cfc5f6_2    conda-forge
  libopenblas               0.3.10               h5a2b251_0
  libpng                    1.6.37               hed695b0_2    conda-forge
  libpq                     12.3                 h5513abc_2    conda-forge
  libsodium                 1.0.18               h516909a_1    conda-forge
  libspatialindex           1.9.3                he1b5a44_3    conda-forge
  libspatialite             5.0.0                h4dde289_0    conda-forge
  libssh2                   1.9.0                hab1572f_5    conda-forge
  libstdcxx-ng              9.3.0               h2ae2ef3_17    conda-forge
  libtiff                   4.1.0                hc7e4089_6    conda-forge
  libuuid                   2.32.1            h14c3975_1000    conda-forge
  libuv                     1.40.0               hd18ef5c_0    conda-forge
  libwebp-base              1.1.0                h516909a_3    conda-forge
  libxcb                    1.13              h14c3975_1002    conda-forge
  libxml2                   2.9.10               h68273f3_2    conda-forge
  libzopfli                 1.0.3                he1b5a44_0    conda-forge
  lxml                      4.6.1                    pypi_0    pypi
  lz4-c                     1.9.2                he1b5a44_3    conda-forge
  markupsafe                1.1.1            py38h8df0ef7_2    conda-forge
  matplotlib-base           3.3.2            py38h4d1ce4f_1    conda-forge
  mistune                   0.8.4           py38h1e0a361_1002    conda-forge
  mpl-scatter-density       0.7                      pypi_0    pypi
  munch                     2.5.0                      py_0    conda-forge
  natsort                   7.0.1                      py_0    conda-forge
  nbclient                  0.5.1                      py_0    conda-forge
  nbconvert                 6.0.7            py38h32f6830_1    conda-forge
  nbdime                    2.1.0                      py_0    conda-forge
  nbformat                  5.0.8                      py_0    conda-forge
  ncurses                   6.2                  he1b5a44_2    conda-forge
  nest-asyncio              1.4.1                      py_0    conda-forge
  nested_dict               1.61               pyh9f0ad1d_0    conda-forge
  networkx                  2.5                        py_0    conda-forge
  nodejs                    14.13.1              h568c755_0    conda-forge
  notebook                  6.1.4            py38h32f6830_1    conda-forge
  numpy                     1.19.2           py38hf89b668_1    conda-forge
  obspy                     1.2.2                    pypi_0    pypi
  olefile                   0.46               pyh9f0ad1d_1    conda-forge
  openjpeg                  2.3.1                h981e76c_3    conda-forge
  openssl                   1.1.1h               h516909a_0    conda-forge
  packaging                 20.4               pyh9f0ad1d_0    conda-forge
  pandas                    1.1.3            py38hc5bc63f_2    conda-forge
  pandoc                    2.11.0.2             hd18ef5c_0    conda-forge
  pandocfilters             1.4.2                      py_1    conda-forge
  parso                     0.7.1              pyh9f0ad1d_0    conda-forge
  patsy                     0.5.1                      py_0    conda-forge
  pcre                      8.44                 he1b5a44_0    conda-forge
  pexpect                   4.8.0              pyh9f0ad1d_2    conda-forge
  pickleshare               0.7.5                   py_1003    conda-forge
  pillow                    8.0.0            py38h9776b28_0    conda-forge
  pip                       20.2.4                     py_0    conda-forge
  pixman                    0.38.0            h516909a_1003    conda-forge
  plotly                    4.11.0                   pypi_0    pypi
  poppler                   0.89.0               h4190859_1    conda-forge
  poppler-data              0.4.9                         1    conda-forge
  postgresql                12.3                 h8573dbc_2    conda-forge
  proj                      7.1.1                h966b41f_3    conda-forge
  prometheus_client         0.8.0              pyh9f0ad1d_0    conda-forge
  prompt-toolkit            3.0.8                      py_0    conda-forge
  pthread-stubs             0.4               h14c3975_1001    conda-forge
  ptitprince                0.2.5                    pypi_0    pypi
  ptvsd                     4.3.2            py38h1e0a361_2    conda-forge
  ptyprocess                0.6.0                   py_1001    conda-forge
  py-tools-ds               0.15.7           py38h32f6830_0    conda-forge
  pycparser                 2.20               pyh9f0ad1d_2    conda-forge
  pygments                  2.7.1                      py_0    conda-forge
  pyhamcrest                2.0.2                    pypi_0    pypi
  pyopenssl                 19.1.0                     py_1    conda-forge
  pyparsing                 2.4.7              pyh9f0ad1d_0    conda-forge
  pyproj                    2.6.1.post1      py38h56787f0_3    conda-forge
  pyrsistent                0.17.3           py38h1e0a361_1    conda-forge
  pyrsr                     0.3.8                      py_0    conda-forge
  pyshp                     2.1.2              pyh9f0ad1d_0    conda-forge
  pysocks                   1.7.1            py38h924ce5b_2    conda-forge
  python                    3.8.6           h852b56e_0_cpython    conda-forge
  python-dateutil           2.8.1                      py_0    conda-forge
  python_abi                3.8                      1_cp38    conda-forge
  pytz                      2020.1             pyh9f0ad1d_0    conda-forge
  pywavelets                1.1.1            py38hab2c0dc_3    conda-forge
  pyyaml                    5.3.1            py38h8df0ef7_1    conda-forge
  pyzmq                     19.0.2           py38ha71036d_2    conda-forge
  readline                  8.0                  he28a2e2_2    conda-forge
  requests                  2.24.0             pyh9f0ad1d_0    conda-forge
  retrying                  1.3.3                    pypi_0    pypi
  rtree                     0.9.4            py38h08f867b_1    conda-forge
  scikit-image              0.17.2           py38hc5bc63f_3    conda-forge
  scikit-learn              0.23.2           py38h519568a_1    conda-forge
  scipy                     1.5.2            py38h8c5af15_2    conda-forge
  seaborn                   0.11.0                        0    conda-forge
  seaborn-base              0.11.0                     py_0    conda-forge
  send2trash                1.5.0                      py_0    conda-forge
  setuptools                49.6.0           py38h924ce5b_2    conda-forge
  shapely                   1.7.1            py38hc7361b7_1    conda-forge
  six                       1.15.0             pyh9f0ad1d_0    conda-forge
  smmap                     3.0.4              pyh9f0ad1d_0    conda-forge
  snappy                    1.1.8                he1b5a44_3    conda-forge
  specclassify              0.2.6              pyh9f0ad1d_0    conda-forge
  spechomo                  0.8.2                      py_0    conda-forge
  spechomo-eval             0.3.3                     dev_0    <develop>
  specidx                   0.2.8                     dev_0    <develop>
  spectral                  0.22.1             pyh9f0ad1d_0    conda-forge
  sqlalchemy                1.3.20                   pypi_0    pypi
  sqlite                    3.33.0               h4cf870e_1    conda-forge
  statsmodels               0.12.0           py38hab2c0dc_1    conda-forge
  tabulate                  0.8.7              pyh9f0ad1d_0    conda-forge
  terminado                 0.9.1            py38h32f6830_1    conda-forge
  testpath                  0.4.4                      py_0    conda-forge
  threadpoolctl             2.1.0              pyh5ca1d4c_0    conda-forge
  tifffile                  2020.10.1                  py_0    conda-forge
  tiledb                    2.1.1                h47b529c_1    conda-forge
  tk                        8.6.10               hed695b0_1    conda-forge
  toolz                     0.11.1                     py_0    conda-forge
  tornado                   6.0.4            py38h1e0a361_2    conda-forge
  tqdm                      4.50.2             pyh9f0ad1d_0    conda-forge
  traitlets                 5.0.5                      py_0    conda-forge
  tzcode                    2020a                h516909a_0    conda-forge
  urllib3                   1.25.11                    py_0    conda-forge
  wcwidth                   0.2.5              pyh9f0ad1d_2    conda-forge
  webencodings              0.5.1                      py_1    conda-forge
  wheel                     0.35.1             pyh9f0ad1d_0    conda-forge
  widgetsnbextension        3.5.1            py38h32f6830_4    conda-forge
  xarray                    0.16.1                   pypi_0    pypi
  xerces-c                  3.2.3                hfe33f54_1    conda-forge
  xeus                      0.24.2               h841dea4_1    conda-forge
  xeus-python               0.8.6            py38h2078d81_1    conda-forge
  xorg-kbproto              1.0.7             h14c3975_1002    conda-forge
  xorg-libice               1.0.10               h516909a_0    conda-forge
  xorg-libsm                1.2.3             h84519dc_1000    conda-forge
  xorg-libx11               1.6.12               h516909a_0    conda-forge
  xorg-libxau               1.0.9                h14c3975_0    conda-forge
  xorg-libxdmcp             1.1.3                h516909a_0    conda-forge
  xorg-libxext              1.3.4                h516909a_0    conda-forge
  xorg-libxrender           0.9.10            h516909a_1002    conda-forge
  xorg-renderproto          0.11.1            h14c3975_1002    conda-forge
  xorg-xextproto            7.3.0             h14c3975_1002    conda-forge
  xorg-xproto               7.0.31            h14c3975_1007    conda-forge
  xz                        5.2.5                h516909a_1    conda-forge
  yaml                      0.2.5                h516909a_0    conda-forge
  zeromq                    4.3.3                he1b5a44_2    conda-forge
  zfp                       0.5.5                he1b5a44_4    conda-forge
  zipp                      3.3.1                      py_0    conda-forge
  zlib                      1.2.11            h516909a_1010    conda-forge
  zstd                      1.4.5                h6597ccf_2    conda-forge
  ```
  
  </details>
  