Show a valid error when "Base Diff Path" is missing.
Review Request #390 — Created May 17, 2008 and submitted
Some users were trying to create review requests against subversion repositories via the web form without specifying a base diff path. This was showing this error: 'UploadDiffForm' object has no attribute 'cleaned_data' This sucks. This change just catches the AttributeError and raises something more meaningful.
Tried adding a diff without specifying the base diff path.
