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