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.