Make undefined variables in debug var=foo more obvious

Fixes #9935

lib/ansible/plugins/action/debug.py

@@ -35,6 +35,8 @@ class ActionModule(ActionBase):
         # FIXME: move the LOOKUP_REGEX somewhere else
         elif 'var' in self._task.args: # and not utils.LOOKUP_REGEX.search(self._task.args['var']):
             results = self._templar.template(self._task.args['var'], convert_bare=True)
+            if results == self._task.args['var']:
+                results = "VARIABLE IS NOT DEFINED!"
             result = dict()
             result[self._task.args['var']] = results
         else: