Improve error message for proto_test. (#5203)
Improve error message for proto_test. (#5203)

Improve error message for proto_test

* Add step for generating protos for RustBoard.
* Add instructions on how to handle new files that must be copied over
to TB for the BUILD to succeed.

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard