Non-existing IP address in database results in message about server problem

@lavamind was testing Exonerator a bit with IP addresses that were definitely not part of our relays fleet or outright non-existing. In those cases a "Server problem" problem is reported with the more detailed description of The database appears to be empty. Please try again later. If this problem persists, please let us know! It seems we need to differentiate better between an actual server problem and IP addresses which just have never been used for relays.