This exception is used by callers for control flow, so it should be a custom exception type, not AssertionError.
https://chromium.googlesource.com/catapult/+/master/dashboard/dashboard/models/internal_only_model.py#26
Fixing this will require auditing all code that transitively calls InternalOnlyModel.get[_async](), which is probably quite a lot of code.
This might be easier to fix after V2SPA launches and replaces most of the request handlers with new api request handlers, which should be less code that is clearer.