zuul-web: config_errors endpoint: return 404 when tenant not found

Fix the endpoint to remove 500 error when tenant is not found.

Change-Id: I1e76d3f120a3d1abec8253f01cd661e0947b3e65

zuul/web/__init__.py

@@ -280,6 +280,8 @@ class ZuulWebAPI(object):
         config_errors = self.rpc.submitJob(
             'zuul:config_errors_list', {'tenant': tenant})
         ret = json.loads(config_errors.data[0])
+        if ret is None:
+            raise cherrypy.HTTPError(404, 'Tenant %s does not exist.' % tenant)
         resp = cherrypy.response
         resp.headers['Access-Control-Allow-Origin'] = '*'
         return ret