Change signature of MultipleFailureException.assertEmpty() to throw Exception

The current signature of AnnotationFormatError() makes it hard to use correctly. If you call it, you have
to declare that you throw Throwable, even though 1) unless someone explicitly throws Throwable or
a creates a subclass of Throwable, the method throws an Error or an Exception, and 2) even if the
method did throw Throwable, there's no reason to declare it.