Return an error when a message can not be delivered to a room

For example, in this case:

Apr 15 18:35:13 ciboule biboumi[18996]: src/bridge/bridge.cpp:156: Cannot send message: no client exist for server irc.quakenet.org

Do not just ignore the message, send an error back.

(from redmine: created on 2015-04-15, closed on 2015-05-11)