more airbreak tweaks

for the mod developer the event is the cleaner approach, from the core developer perspective it's ugly: Whenever I trigger this event I have to set an instance variable beforehand to pass the exception object. I guess our policy is to sacrifice the core's elegance to the benefit of the mod's usability. But it's not a perfect solution for the mod developer as well. When you use this callback it's likely that you want to do something with the exception object. So you have to figure out that there is a kind of global object (I think I will name it @exception)  and you have to trust it that it will contain the right value when the callback is triggered. An event argument would be more natural. But we already have this "revise event api" on the agenda ...