Use 'except ... as' syntax

This syntax works on Python 2.6 through 3.x.  lib/ansible/module_utils
(and lib/ansible/modules) need to support Python 2.4, so I didn't touch
those.

lib/ansible/plugins/lookup/nested.py

@@ -32,7 +32,7 @@ class LookupModule(LookupBase):
         for x in terms:
             try:
                 intermediate = listify_lookup_plugin_terms(x, templar=self._templar, loader=self._loader, fail_on_undefined=True)
-            except UndefinedError, e:
+            except UndefinedError as e:
                 raise AnsibleUndefinedVariable("One of the nested variables was undefined. The error was: %s" % e)
             results.append(intermediate)
         return results