Improve error handling

[jamescooke]

At the moment, as soon as an error is found, processing stops. However, there can be multiple errors in a single test - therefore, error handling must be improved to pass multiple errors back to Flake8 where possible.

There are two classes of errors in Flake8-AAA:

• Halting errors: no further linting is possible when these occur:

  • AAA01: no Act block found in test
  • AAA02: multiple Act blocks found in test
  • AAA99: collision when marking this line as NEW_CODE, was already OLD_CODE

• Non-blocking errors: can occur 1 or more times in a single test and do not prevent other errors from happening (all other errors):

Tasks

• Adjust Function.check_all() to return iterable / generator of non-blocking errors.
• Ensure that this change works with Function.get_errors() and do_command_line().

[jamescooke]

Should be done before any changes discussed in #62 .