new feature: put exception as a param to event_failed

[lksv]

I may want to propagate some exceptions and others I may want to handle in event_failed callback.

I am not sure whether catching exception at all is good idea. Maybe best solution would be to re-raise exception after calling event_failed.

[troessner]

Sorry for the late reply, I kind of missed this one!

I may want to propagate some exceptions and others I may want to handle in event_failed callback.

Yes, makes sense. Care for a pull request? ;)

I am not sure whether catching exception at all is good idea. Maybe best solution would be to re-raise exception after calling event_failed.

I'm not so sure about this. But then again, if we pass the exception to event_failed it's up to you if you want to just re-raise yourself at the end of the callback. What do you think?