code: func tumbleNow(c context.Context, m tumble.Mutation) error {
  	err := tumble.RunMutation(c, m)
  	if grpc.Code(err) == codes.Unknown {
  		logging.WithError(err).Errorf(c, "unknown error while applying mutation %v", m)
  		err = grpcutil.Internal
  	}
  	logging.Fields{"root": m.Root(c)}.Infof(c, "tumbleNow success")
  	return err
  }
docs: // tumbleNow will run the mutation immediately, converting any non grpc errors
  // to codes.Internal.