Drop event listener exceptions on the floor.

We get a lot of failures from assertions that event listeners haven't
thrown exceptions. As the FIXME above these assertions suggests, we
should just drop them on the floor.
auto merge of #2582 : zwarich/servo/drop-exceptions, r=jdm

We get a lot of failures from assertions that event listeners haven't
thrown exceptions. As the FIXME above these assertions suggests, we
should just drop them on the floor.