Generator Bugs

[DavidMinarsch]

Is your feature request related to a problem? Please describe.
A small bugs are present in the generator:

• errors in the specification cause a stack trace rather than just a clean warning message
• the generated files are not linted with black
• if the message defines speech act contents already used by the default properties (e.g. target, body), then the generator does not show this as an error
• optional fields not decoding: What is the correct way in proto3 to use HasField? googleapis/google-cloud-python#1402

• repeat usage of variables ("key, value"). Mypy complains and we should use unique and descriptive names for these variables:
  

• custom types not correctly declared when tac protocol is generated:
  

• in serialisation.py: 'packages.fetchai.protocols.tac.custom_types.ErrorInfo' imported but unused

• pt:list[pt:str] generates a tuple in oef protocol. explain this in the docs!

• add type declaration in decoding logic or serialisation performative_content = dict() # type: Dict[str, Any]

Describe the solution you'd like
Fix the above.