during a recent chat I had with the good Sergey (on another topic), I
have found that the JAX-RS's standard WebApplicationException (and child
exceptions) are handled by CXF, according to the specs, via
Syncope's RestServiceExceptionMapper, however, contains some code  to
manage such exceptions, which is never going to be triggered, at this point.
Please consider that this affects both 1.2 and 2.0.
IMO, we should either:
(a) remove  and let CXF do its work according to the specs
(b) set the 'default.wae.mapper.least.specific' (which is false by
default) to delegate WebApplicationException to 
Tirasa - Open Source Excellence
Member at The Apache Software Foundation
Syncope, Cocoon, Olingo, CXF, OpenJPA, PonyMail