Two error codes are renamed

A command whose timeout outlasts the sandbox's time limit or its funded time is refused with 409 time_limit_too_short, and details.secondsLeft says how long is left: time_limit_too_short replaces lease_too_short. A host that is busy answers 503 busy, safe to retry with the same key: busy replaces fence_conflict. Code that matched the old names should match the new ones. See errors.