Make consensus GETINFO return 551 when using microdescs
The tor control-spec says that 551 should be returned for "dir/status" queries where microdescs are in use, and so full descriptors are unavailable. https://gitweb.torproject.org/torspec.git/tree/control-spec.txt#n737
This doesn't happen for dir/status-vote/current/consensus when tor only has a microdesc consensus, instead, tor returns "552 Unrecognized key "dir/status-vote/current/consensus". https://gitweb.torproject.org/tor.git/tree/src/or/control.c#n2004
The 552 is misleading and not to spec, instead, we should return 551 with a helpful error message.
Credit to s0rlxmh0 for reporting this issue.