Added some useful error information for empty diffs and invalid repositories.

Review Request #1487 — Created March 28, 2010 and submitted

Information

RBTools

Reviewers

Added some useful error information for empty diffs and invalid repositories.

Our errors to date in RBTools have been less than useful. This change provides
more detailed errors for two common cases: Empty diffs, and invalid
repositories.

The new text makes it more clear what went wrong, and offers options for
fixing or investigating the problem.
Tested both cases against a local repository. I was able to reproduce both scenarios and see the expected output.