When file transforms race and lose, accept defeat gracefully

Summary: Fixes T8277. Transforming files can race; resolve the race after we lose.

Test Plan:

• Added sleep(10) near the bottom of the transform controller.
• Transformed a file in two browser windows at the same time; got something like this (exception corresponds to the loser of the race):

{F412526}

• Applied patch.
• Repeated process, got this:

{F412527}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8277

Differential Revision: https://secure.phabricator.com/D12965