Slightly improve error message when given incorrect module path

[TimFelixBeyer]

Improve error message when given incorrect module path

Currently, python returns a helpful error message when users accidentally try to run python -m and include a '.py' ending in the path.

user@machine:~$ python3 -m path.to.file.py
>>> Error while finding module specification for 'path.to.file.py' 
(ModuleNotFoundError: No module named 'path'). 
Try using 'path.to.file' instead of 'path.to.file.py' as the module name.

However, a more common scenario for inexperienced users would be using the path to the file directly:

user@machine:~$ python3 -m path/to/file.py
>>> Error while finding module specification for 'path/to/file.py' 
(ModuleNotFoundError: No module named 'path/to/file'). 
Try using 'path/to/file' instead of 'path/to/file.py' as the module name.

This PR changes the error message to the more helpful:

user@machine:~$ python3 -m path/to/file.py
>>> Error while finding module specification for 'path/to/file.py' 
(ModuleNotFoundError: No module named 'path/to/file'). 
Try using 'path.to.file' instead of 'path/to/file.py' as the module name.

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