Fix review-requests/ resource to catch diff related errors.
Fix review-requests/ resource to catch diff related errors.

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/

bkochendorfer/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,KnowNo/reviewboard,brennie/reviewboard,KnowNo/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,davidt/reviewboard,beol/reviewboard,KnowNo/reviewboard,1tush/reviewboard,1tush/reviewboard,1tush/reviewboard,1tush/reviewboard,1tush/reviewboard,davidt/reviewboard,1tush/reviewboard,1tush/reviewboard,brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,beol/reviewboard,brennie/reviewboard,davidt/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,custode/reviewboard,custode/reviewboard,bkochendorfer/reviewboard,chipx86/reviewboard,beol/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,sgallagher/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,custode/reviewboard,1tush/reviewboard,beol/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,davidt/reviewboard,brennie/reviewboard,custode/reviewboard