restapi: Return additional info for 400 messages

This was already done for JAXB exceptions, but is missing for RESTEasy
JAXB marshall exceptions.

Change-Id: I31a7a5f487c662cbcba3734153e9fcb120ba330d
Bug-Url: https://bugzilla.redhat.com/867794
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>