Unexpected AuthError from _check_event_auth

[richvdh]

#11011 added a check, boldly asserting that some code was unreachable. The evidence suggests otherwise. We should find out why this is happening.

[richvdh]

Example stacktrace:

AuthError: 403: Auth events could not be found
  File "synapse/handlers/federation_event.py", line 983, in _process_received_pdu
    context,
  File "synapse/handlers/federation_event.py", line 1371, in _check_event_auth
    origin, event
  File "synapse/handlers/federation_event.py", line 1683, in _load_or_fetch_auth_events_for_event
    raise AuthError(code=HTTPStatus.FORBIDDEN, msg="Auth events could not be found")

https://sentry.matrix.org/sentry/synapse-matrixorg/issues/233645
https://sentry.matrix.org/sentry/synapse-modular/issues/235420

[richvdh]

#11001 made AuthError a documented exception from _check_event_auth, so the problem is now simply that we log it. Fixed by #13145.