text: Title: better error message when NotebookRunner is initialized with a string instead of a path
  Body: `NotebookRunner` expects a string with Python code or a `pathlib.Path` object to a `.py` file. However, new users may do this:
  
  ```python
  NotebookRunner('script.py')
  ```
  
  But instead they should do:
  
  ```python
  from pathlib import Path
  
  NotebookRunner(Path('script.py'))
  ```
  
  If passed a string, we should try to determine if the arguments "looks like a path to a file", and raise an error accordingly: "perhaps you meant passing a pathlib.path object".
  
  