Show a clearer error when an UnknownHostException is due to a node referral

Crowd shows an UnknownHostException when the hostname given for a connection can't be resolved, but also when a node referral points to a host that can't be resolved. It would be better to make a clear distinction, so users know that turning off "Use Node Referrals" may help them to use that directory.

Detection would need to be a heuristic.