Make it an error to try to hit a change handler that doesn't exist

lib/ansible/playbook.py

@@ -338,16 +338,20 @@ class PlayBook(object):
         # for all registered handlers in the ansible playbook
         # for this particular pattern group
 
+        found = False
         for x in handlers:
             name = x.get('name', None)
             if name is None:
                 raise errors.AnsibleError('handler is missing a name')
             if match_name == name:
+                found = True
                 self.callbacks.on_notify(host, name)
                 # flag the handler with the list of hosts it needs to be run on, it will be run later
                 if not 'run' in x:
                     x['run'] = []
                 x['run'].append(host)
+        if not found:
+            raise errors.AnsibleError("change handler (%s) is not defined" % match_name)
 
     # *****************************************************