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.

Unit tests pass.
The code I was writing that hit this no longer had an issue importing.