Break a circular reference in diffutils.

diffutils imports from Profile, which eventually imports diffutils
again, through a chain of imports. Depending on what's been imported
first, things are fine, and in fact, Review Board itself has no problems
with this. However, an extension trying to import from diffutils will
end up hitting this issue.

We only had the Profile import for the purpose of the DoesNotExist
exception. Instead, we can easily replace that with the more generic
ObjectDoesNotExist, breaking this cycle.

Reviewed at https://reviews.reviewboard.org/r/5839/