When failing because of vars_files templating, try and bubble up the file/line info

lib/ansible/vars/__init__.py

@@ -247,7 +247,7 @@ class VariableManager:
                         raise AnsibleError("vars file %s was not found" % vars_file_item)
                 except (UndefinedError, AnsibleUndefinedVariable):
                     if host is not None and self._fact_cache.get(host.name, dict()).get('module_setup') and task is not None:
-                        raise
+                        raise AnsibleUndefinedVariable("an undefined variable was found when attempting to template the vars_files item '%s'" % vars_file_item, obj=vars_file_item)
                     else:
                         # we do not have a full context here, and the missing variable could be
                         # because of that, so just show a warning and continue