spec change: libmsg/ CmdList::error() removed (Error is not only InvalidCMD but InvalidID);
spec change: libmsg/ Msg.usage() takes errorcode instead of option list;
lib*cmd/ distinct error code at test module: InvalidID=1, InvalidCMD=2
debug: rescue InvalidID also catches InvalidCMD -> InvalidCMD must be located prior to InvalidID;
update usage sentence;