Better response to "Unknown MODE flag" response

[henk84]

WARN -- rfc2812.rb:1393:in `process': Unknown message ":irc-02.netwichtig.ch 501 marvin :Unknown MODE flag"

rbot should handle this message IMHO with a better log entry showing what command lead to this response, giving some indication what mode is not supported.
Depending on what mode rbot tries to set and why, it might be a good idea to do something else, e.g. abandon the current operation if setting a mode that is required for it could not be set.