Informative error message on not being able to locate GNU diff in a Perforce workspace

Review Request #2685 — Created Nov. 4, 2011 and submitted — Latest diff uploaded

Information

RBTools
release-0.3.x

Reviewers

Currently, if no GNU diff in the path, post-review gives an unhandled exception and a call stack. It scares the user, and left him without any clue what is wrong.
I found this is solved in case of SVN so I ported that solution to Perfoce too.
(This patch sould be applied to the master too.)