Add option to fail on undefined variables to listify

And use it in the call to get the loop items for a task.

lib/ansible/plugins/lookup/nested.py

@@ -17,22 +17,29 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
-from ansible.errors import AnsibleError
+from jinja2.exceptions import UndefinedError
+
+from ansible.errors import AnsibleError, AnsibleUndefinedVariable
 from ansible.plugins.lookup import LookupBase
 from ansible.utils.listify import listify_lookup_plugin_terms
 
 class LookupModule(LookupBase):
 
-    def __lookup_variabless(self, terms, variables):
+    def __lookup_variables(self, terms, variables):
+        foo = variables.copy()
+        foo.pop('vars')
         results = []
         for x in terms:
-            intermediate = listify_lookup_plugin_terms(x, variables, loader=self._loader)
+            try:
+                intermediate = listify_lookup_plugin_terms(x, variables, loader=self._loader, fail_on_undefined=True)
+            except UndefinedError, e:
+                raise AnsibleUndefinedVariable("One of the nested variables was undefined. The error was: %s" % e)
             results.append(intermediate)
         return results
 
     def run(self, terms, variables=None, **kwargs):
 
-        terms = self.__lookup_variabless(terms, variables)
+        terms = self.__lookup_variables(terms, variables)
 
         my_list = terms[:]
         my_list.reverse()