Catch various SCMErrors in the ValidateDiffResource
The diff validation was catching a variety of errors, but the scmtools errors

weren't part of that. This wasn't an issue when I first wrote it but subsequent

clean-up has changed that. I've added handling for the ShortSHA1Error specifically

(to make it match what the front-end code expects), and a generic catch-all for

SCMError.

Ran unit tests.