Returning MENU_NOT_FOUND does not always render a proper 404 page

[quicksketch]

Describe your issue or idea

In #100, we found a problem where returning MENU_NOT_FOUND in node_page_view() did not render a full 404 page with the title, layout, and blocks. Instead it only returned a page with nothing but the string "404".

Steps to reproduce

With the latest 1.x (or after 1.11.0 comes out), modify node_page_view() and replace the current page_not_found() call:

Replace:

  if ($type->settings['hidden_path'] && !$bypass_hidden_path) {
    backdrop_not_found();
    backdrop_exit();
  }

With:

  if ($type->settings['hidden_path'] && !$bypass_hidden_path) {
    return MENU_NOT_FOUND;
  }

Then disable path access to that content type and view as an anonymous user.

Actual behavior

You should get a fully themed 404 page.

Expected behavior

You get a page with nothing but "404" printed on it.

Relevant version/system information

Backdrop 1.11.0 (currently only Backdrop 1.x dev version).