R1:c1bff3b8013e

Add an "Restartable: If Failed" behavior to Harbormaster build plans

Summary:
Ref T13249. Ref T13258. In some cases, builds are not idempotent and should not be restarted casually.

If the scary part is at the very end (deploy / provision / whatever), it could be okay to restart them if they previously failed.

Also, make the "reasons why you can't restart" and "explanations of why you can't restart" logic a little more cohesive.

Test Plan:
- Tried to restart builds in various states (failed/not failed…