Raise a warning if javax.annotation.Priority cannot be found

When using Weld on Java 9 and the java.xml.ws.annotation module is enabled, interceptors won't be registered. The reason being, that this module contains only a subset of the Commons Annotation API, in particular, @Priority isn't there. The code (AnnotationApiAbstraction) for retrieving inceptor priorities in Weld is very defensive, it just returns null if the annotation type isn't present, causing interceptors not being picked up.

Having a warning about that annotation type (and potentially others) missing, would help users to understand that very subtle issue. Currently, interceptors just won't be applied without any error or warning whatsoever.