Is that really any simpler or clearer than:

  <script>
    ...
  </script>
  <noscript>
    ...
  </noscript>

I think something like:

    <script src="..." id="bla></script>
    <noscript for="bla">Script not loaded</noscript>

Would be more flexible, and would be consistent with how labels, datalists, etc. are linked to their element.

Yes! That's what I meant. Wasn't able to express it clearly but you did :)