GuardMgr: Improve and revamp error types and messages.

This branch works on improving the error messages from guardmgr, particularly in the case where we don't have any viable guards or fallbacks. (Experience in Tor suggests that this information is useful to have when diagnosing this error.) It involves a bit of bookkeeping and helper types, but the change actually makes some of the code a bit cleaner IMO.

Part of #323 (closed)