API improvements

[pheel]

Proposed changes:

• When Rasa is run with --enable-api, exceptions incurred as a result of a request are redirected to the HTTP response. This is nice, but it's useful to also have them show up in ones favorite stream handler via the Python logger.
• When attempting to load an outdated model, Rasa terminates with an UnsupportedModelError or UnsupportedDialogueModelError. This is problematic for users that have access to the /train route, but not the file system from which models are retrieved. This allows Rasa to boot so that such users can retrain.

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)

[sara-tagger]

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

[wochinge]

Thanks for your contribution! Super cool 🚀

[wochinge]

Thanks a lot!