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, 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.)

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13258, T13249

Differential Revision: https://secure.phabricator.com/D20252