Don't throw a 500 for DiffViewer with a deleted version (Bug 855894)

After merging https://github.com/mozilla/zamboni/commit/bc91059b804ff82dd769b75e5f8cc377aee3a4dc, we learned that both the file view and the compare file view were susceptible to throwing a traceback when  viewing a file that has had its version deleted.

This should address that and fix the test that broke.