arti-client: Unlock the state manager on failure to bootstrap

StateMgr got a new unlock() method that does what it says on the tin. We now call it from bootstrap() using the new util::StateMgrUnlockGuard, which works in a manner similar to the BoolResetter from tor_dirmgr.

(A decent small little task in future might be to unify these types in some sort of general arti utility crate?)

closes #335 (closed)