Submitting Challenges Without a Flag Errors

[sentry]

Sentry Issue: BACKEND-4A

TypeError: string indices must be integers
(8 additional frame(s) were not displayed)
...
  File "rest_framework/views.py", line 466, in handle_exception
    response = exception_handler(exc, context)
  File "backend/exception_handler.py", line 50, in handle_exception
    raise exc
  File "rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "challenge/views.py", line 302, in post
    if not challenge.flag_plugin.check(flag, user=user, team=team):
  File "plugins/flag/plaintext.py", line 9, in check
    return self.challenge.flag_metadata["flag"] == flag

Unsure if this is something we actually want to catch. The self_check method does already cover this but we should probably make it clearer what's happened.