The diff fragments view logs a traceback when it hits an error, but we

don't always want this. If the review request exists but the files

backing its diff are now gone, every access would result in a very

lengthy traceback.
We no longer log File Not Found errors, instead just displaying the File

Not Found on the page instead. This will cut down a lot on noisy logs

for review requests like this.

Tested with and without File Not Found errors and other exceptions.

Verified that we weren't getting log spam from File Not Founds.