Throwing DescriptorUnavailable when tor can't provide one for a relay

Tor gives a pretty sucky 'GETINFO request contained unrecognized keywords:
ns/id/5AC9C5AA75BA1F18D8459B326B4B8111A856D290' error when the descriptor is
unavailable for a given relay. Having us give something better...

  https://trac.torproject.org/projects/tor/ticket/13879