Nickname registration message is confusing

toralf says:

wrt to this syslog massage : "You specified a server "zwiebeltoralf" by name, but the directory authorities do not have any key registered for this nickname"    - how can I register my nickname ?

We should change this message to be clearer, particularly because we don't use the Named/Unnamed flags any more.

We could just say "don't have any relay for this nickname".