Warn about unused event handler return 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.

event-bus-proc/src/main/java/dev/kske/eventbus/proc/EventProcessor.java

@@ -63,6 +63,10 @@ public class EventProcessor extends AbstractProcessor {
 			else
 				pass = true;
 
+			// Warn the user about unused return values
+			if (useParameter && eventHandler.getReturnType().getKind() != TypeKind.VOID)
+					warning(eventHandler, "Unused return value");
+
 			// Abort checking if the handler signature is incorrect
 			if (!pass)
 				continue;