Show a valid error when "Base Diff Path" is missing.

Review Request #390 — Created May 17, 2008 and submitted — Latest diff uploaded

Information

Review Board SVN (deprecated)
trunk
460

Reviewers

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.