When polymorphism or a priority is defined at listener-level and then
identically defined for a specific handler, a warning is issued.
This does not affect redefinitions of default values.
If an event handler has a non-void return type, a warning is issued as
the event bus cannot use the returned value.
In rare cases this might be justified as the event handler could be
invoked directly.
When encountering an event handler with an invalid signature, the
processor doesn't crash anymore. Also, event parameters that aren't
objects are now reported as errors.
