Fix AttributeError when trying to accessing invalid diff fragment.

When browsing to an invalid FileDiff via, we would raise a 404 exception but
this was getting caught prematurely and eventually causing a server error. This
change simply passes Http404 errors back up the stack.

Testing Done:
Opened an invalid fragment URL and saw a 404 page instead of a traceback.

Bugs closed: 3473

Reviewed at https://reviews.reviewboard.org/r/6089/