Add some logging at a point that looks unreachable

changelog.d/11011.misc

@@ -0,0 +1 @@
+Clean up some of the federation event authentication code for clarity.

synapse/handlers/federation_event.py

@@ -894,6 +894,9 @@ async def _process_received_pdu(
  backfilled=backfilled,
  )
  except AuthError as e:
+ # FIXME richvdh 2021/10/07 I don't think this is reachable. Let's log it
+ # for now
+ logger.exception("Unexpected AuthError from _check_event_auth")
  raise FederationError("ERROR", e.code, e.msg, affected=event.event_id)
 
  await self._run_push_actions_and_persist_event(event, context, backfilled)