fixes #1037

The value passed to extends, import, or include could be a variable, which could be undefined. Some loaders would incidentally trigger the UndefinedError, while others such as DictLoader would not, producing an unhelpful TemplateNotFound message.

Passing Undefined to get_template raises an UndefinedError. If select_template raises TemplatesNotFound, any undefined values in the list of attempts show as their undefined message rather than the empty string. get_or_select_template passes Undefined to get_template.