message: add case when errors is garbage collected
diff: @@ -150,17 +150,22 @@ class Server(object):
  self.start()
  def __del__(self):
  + # At shutdown, `errors` may have been garbage collected.
  + if errors is not None:
  + exception = errors.UnimplementedError
  + else:
  + exception = Exception
  try:
  c_api.TF_ServerStop(self._server)
  # Clean shutdown of servers is not yet implemented, so
  # we leak instead of calling c_api.TF_DeleteServer here.
  # See:
  # https://github.com/tensorflow/tensorflow/blob/0495317a6e9dd4cac577b9d5cf9525e62b571018/tensorflow/core/distributed_runtime/rpc/grpc_server_lib.h#L73
  - except errors.UnimplementedError:
  - pass
  except AttributeError:
  # At shutdown, `c_api` may have been garbage collected.
  pass
  + except exception:
  + pass
  self._server = None
  def start(self):
  