Improve exception reporting for CLI commands

[tmbo]

Motivation:

• We print unecessary stack traces for exceptions like failures to read a training data yaml file. This isn't helpful as we know that the issue is the syntax in the yaml file and a stack trace won't help.
• Many exceptions reported to sentry are things we expect to happen (errors in the training data format, a wrong action name, ...)

Before:

After:

Proposed changes:

• base class for all exceptions raised in Rasa Open Source that are "expected" (e.g. file format issues, missing action names, ... things we expect a user to get wrong. they will still bubble up, but are not going to create a stacktrace). RasaOpenSourceException
• all RasaOpenSourceExceptions will be caught if thrown as part of a CLI command and stack traces will not be printed by default, instead we use print_error
• changed NameError exception raised when an action can not be found to be a ValueError instead
• raise InvalidYAMLFileException instead of YAMLError (from ruamel) when failing to load a yaml file
• made InvalidDomain a RasaOpenSourceException
• introduced MissingDependencyException as an exception raised if packages need to be installed
• added line numbers to output of YAML validation to the command line as paths on their own are hard to navigate:

InvalidYamlFileError: Failed to validate 'data/nlu.yml'. Please make sure the file is correct and all mandatory parameters are specified. Please take a look at the errors found during validation:
  in data/nlu.yml:2:
      Key 'ZZZZ' was not defined. Path: '/nlu/0'

• properly exit on ctrl+c / ctrl+d in rasa shell nlu command
• RasaOpenSourceExceptions are ignored by sentry

TODO

• InvalidRule is very user-unfriendly and vague, but out of this PRs scope

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)

[m-vdb]

🚀 🚀 🚀 Amazing, thanks for doing these changes. I definitely think that they will improve the UX

[tmbo]

thanks a lot for the quick review super helpful to get this merged in time ⭐

[m-vdb]

Great 💯 I think it's mergeable once the tests go green 🟢