misleading error message when base template has no blocks

[James Tauber]

If one extends a template that has no blocks, the error shown is a IndexError. It would be nice if that case were caught and a message to the effect of "base template has no blocks" shown.

For example, change render() in loader_tags.py to catch IndexError:

        try:
            while isinstance(compiled_parent.nodelist[pos], TextNode):
                pos += 1
        except IndexError:
            raise TemplateSyntaxError, "Template cannot be extended, because it doesn't contain a TextNode" 

[James Bennett]

Duplicate of #6558.