Adding error handler to catch SystemErrors. Now the bot catches and logs them and tries to ignore them.
Though the SystemErrors are only seen in the mainloop (run) and the fixroomstatus loop, they may also occur in fixgrouproomstatus, so catch them there too.
Note this commit does not fix issue #238, but implements a workaround for it: it discards the stanza and continues with its business, hoping nothing goes wrong...
Closes #238