show callback exception on debug

lib/ansible/executor/task_queue_manager.py

@@ -320,3 +320,6 @@ class TaskQueueManager:
                 except Exception as e:
                     #TODO: add config toggle to make this fatal or not?
                     display.warning(u"Failure using method (%s) in callback plugin (%s): %s" % (to_unicode(method_name), to_unicode(callback_plugin), to_unicode(e)))
+                    from traceback import format_tb
+                    from sys import exc_info
+                    display.debug('Callback Exception: \n' + ' '.join(format_tb(exc_info()[2])))