Provide a sane error if encountering multiple conflicting repositories.

Review Request #7459 — Created June 26, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
7f1f601...

Reviewers

If two repositories conflict (one has a Mirror Path matching the Path of
another repository), then validating a diff or creating a review request
in the API will result in an HTTP 500 error, due to a
MultipleObjectsReturned exception.

We now check for this and provide a more reasonable error message
instead, with a suggestion for how to resolve the issue.

Unit tests pass.