R1:d70ca6b7c8b5

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…