When an exception occurs during the execution of an event handler, it is
caught, wrapped inside an exception event and dispatched on the event
bus.
This applies to any throwable, but is not very useful for errors, as
these are not normally caught. Assertion errors in particular, which are
used in unit tests, should not be caught, as this would cause the test
runner to miss a failed test.
Therefore, errors are now transparently passed through to the caller of
the dispatch method.
