Undefined event listener results in "Property 'handleEvent' is not callable" error

There was a bug in our application where we were passing undefined as a handler to addEventListener. Raven.js was wrapping this in an object before adding the event listener, so an object without a handleEvent function was being attached. In Firefox, this results in an exception with no stacktrace. Ideally Firefox would log a warning when this happens, but Raven.js could do this as well.