log exception on invalid jenkins notifications

I have been playing with zuul on port 8001 using curl:

  curl http://127.0.0.1:8001/jenkins_endpoint

Had to `strace -f` the process to find out what was going wrong. Logging
the exception will simply make it easier to track possible issues coming
from the Jenkins Notification plugin..

Change-Id: I461146d0a6e7d63a0d8bedc3d4a6ad19e22548d6
Reviewed-on: https://review.openstack.org/13817
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins

zuul/launcher/jenkins.py

@@ -64,7 +64,11 @@ class JenkinsCallback(threading.Thread):
             return ['Zuul good.']
 
     def jenkins_endpoint(self, request):
-        data = json.loads(request.body)
+        try:
+            data = json.loads(request.body)
+        except:
+            self.log.exception("Exception handling Jenkins notification:")
+            raise  # let wsgi handler process the issue
         if data:
             self.log.debug("Received data from Jenkins: \n%s" %
                            (pprint.pformat(data)))