Pickling exceptions

[Mazyod]

Proposed changes:

• Fix error thrown when attempting to pickle UnsupportedModelError and InvalidModelError

I needed to pass exceptions from background processes to be handled in the main process, but it was these exceptions were causing issues during pickling. The fix is easy, and hopefully a better practice especially for a project like rasa which relies on multiprocessing.

Previous error:

def test_exception_pickling():
        exception = UnsupportedModelError("test run")
>       cycled_exception = pickle.loads(pickle.dumps(exception))
E       TypeError: __init__() missing 1 required positional argument: 'message'

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[Mazyod]

Another option is to make message an optional argument:
https://stackoverflow.com/a/16245182/456434

[sara-tagger]

Thanks for submitting a pull request 🚀 @tttthomasssss will take a look at it as soon as possible ✨

[Mazyod]

would appreciate any feedback on this PR

[tttthomasssss]

Hey, apologies this took so long. Thanks very much for submitting this PR.

All looks good codewise - great work 🎸 ! Could you just expand on the purpose of being able to pickle the exceptions in the changelog? I.e. just a line or two on why this is needed and what use-case it covers.

[Mazyod]

Appreciate it @tttthomasssss. Certainly, I have added the purpose for making exceptions pickleable. I hope the explanation would suffice, otherwise I could try adding more information.

[tttthomasssss]

Awesome, thanks very much for the lightning fast resolution 🚀