Add an "Restartable: If Failed" behavior to Harbormaster build plans
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.
- Tried to restart builds in various states (failed/not failed, restartable always/if failed/never, already restarted), got appropriate errors or restarts.
- (I'm not sure the "Autoplan" error is normally reachable, since you can't edit autoplans to configure things to let you try to restart them.)
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20252