text: Title: Better error message when missing __init__.py
  Body: If a dotted path fails to import, we should check if the reason is a missing `__init__.py` file and suggest adding it. Current error:
  
  ```pytb
  Traceback (most recent call last):
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/cli/io.py", line 20, in wrapper
      fn(**kwargs)
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/cli/task.py", line 34, in main
      dag, args = _custom_command(parser)
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/cli/parsers.py", line 405, in _custom_command
      dag, args = load_dag_from_entry_point_and_parser(entry_point, parser,
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/cli/parsers.py", line 451, in load_dag_from_entry_point_and_parser
      dag, args = _process_file_dir_or_glob(parser)
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/cli/parsers.py", line 392, in _process_file_dir_or_glob
      dag = DAGSpec(dagspec_arg).to_dag()
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/spec/dagspec.py", line 424, in to_dag
      dag = self._to_dag()
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/spec/dagspec.py", line 473, in _to_dag
      process_tasks(dag, self, root_path=self._parent_path)
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/spec/dagspec.py", line 745, in process_tasks
      task, up = task_dict.to_task(dag)
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/spec/taskspec.py", line 298, in to_task
      return _init_task(data=data,
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/spec/taskspec.py", line 381, in _init_task
      task.on_finish = dotted_path.DottedPath(on_finish,
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/util/dotted_path.py", line 47, in __init__
      self._load_callable()
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/util/dotted_path.py", line 54, in _load_callable
      self._callable = load_callable_dotted_path(self._spec.dotted_path)
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/util/dotted_path.py", line 169, in load_callable_dotted_path
      loaded_object = load_dotted_path(dotted_path=dotted_path,
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/site-packages/ploomber/util/dotted_path.py", line 128, in load_dotted_path
      module = importlib.import_module(mod)
    File "/Users/Edu/miniconda3/envs/ml-testing/lib/python3.9/importlib/__init__.py", line 127, in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
    File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
    File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
    File "<frozen importlib._bootstrap>", line 984, in _find_and_load_unlocked
  ModuleNotFoundError: An error happened when trying to import dotted path "tests.quality.clean": No module named 'tests.quality'
  ```