Return an error when trying to POST a diff on a review request with no repository

Review Request #5522 — Created Feb. 21, 2014 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
5f40329...

Reviewers

The diff-list API didn't check that the associated ReviewRequest had a
repository before it started trying to parse things and validate the uploaded
data. This meant that trying to add a diff (likely via rbtools) to an
attachments-only review request would cause an exception on the server.

I've added a test for this, and fixed the bug. While I was trying to decide on
the right WebAPIError to use, I noticed an ugly misalignment in errors.py, so
I've fixed that too.

Ran unit tests.