Backend()'s "dynamic backends disallowed" error is swallowed as bare undefined without --enable-experimental-top-level-await

[TartanLlama]

function registerBackend(name, target, hostOverride) {
  try {
    new Backend({ name, target, ...(hostOverride && { hostOverride }) });
    return null;
  } catch (e) {
    if (String(e.message).toLowerCase().includes("not enabled")) return 403;
    return 409;
  }
}

async function handleRequest(req) {
  const status = registerBackend(/* ... */);
  if (status !== null) return new Response(null, { status });
  // ...
}

addEventListener("fetch", (event) => {
  event.respondWith(handleRequest(event.request));
});

This should return 403 if dynamic backends are not enabled, but currently returns 500 with no error message without --enable-experimental-top-level-await