message: Re-added missing exception handling after rebase
diff: @@ -317,7 +317,10 @@ class Coordinator:
  not isinstance(message, coordinator_pb2.RendezvousRequest)
  and participant_id not in self.participants.participants.keys()
  ):
  - raise Exception
  + raise UnknownParticipantError(
  + f"Unknown participant {participant_id}. "
  + "Please try to rendezvous with the coordinator before making a request."
  + )
  # pylint: disable-msg=no-else-return
  if isinstance(message, coordinator_pb2.RendezvousRequest):
  