Detect if a commit *really* doesn't exist and 4oh4 from Diffusion commit view
Summary: rather than showing an erroneous "we still parsing" message.
Test Plan: for each version control system, typed in a garbage URL and got a 4oh4. (note this actually fails for SVN -- see comment about how my code fails atm -- and DiffusionGitRequest seems to pick off this error in advance and returns an AphrontUsageException.)
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T1624
Differential Revision: https://secure.phabricator.com/D3201