It seems like this would fail for blocks inside sections inside parent tags:

```html+jinja
{{< foo }}
  {{# bar }}
    {{$ baz }}{{/ baz }}
  {{/ bar }}
{{/ foo }}
```

… which probably is a perfectly valid failure case, since the contents of parent tags should really just be the blocks to be overridden. But if that's the case, do we want to throw an exception when parsing if a block arg is ever nested inside something besides a parent tag? Or is it better to let things fail and not tell people why?