Fix review-requests/ resource to catch diff related errors.
Review Request #4763 — Created Oct. 16, 2013 and submitted
There were several diff parser related exceptions that weren't being handled in
the review requests resource, which would cause a 500. This change catches
those errors and translates them into equivalent API errors.
Testing done: Ran unit tests.
Reviewed at https://reviews.reviewboard.org/r/4754/