Delay calling onComplete for inline module script parse errors

Fixes #9864.

source

@@ -106840,7 +106840,15 @@ document.querySelector("button").addEventListener("click", bound);
      <li><p>Set <var>moduleScript</var>'s <span data-x="concept-script-error-to-rethrow">error to
      rethrow</span> to <var>moduleScript</var>'s <span>parse error</span>.</p></li>
 
-     <li><p>Run <var>onComplete</var> given <var>moduleScript</var>.</li>
+     <li>
+      <p><span>Queue a microtask</span> to run <var>onComplete</var> given
+      <var>moduleScript</var>.</p>
+
+      <p class="note">We queue a microtask, instead of running <var>onComplete</var> immediately,
+      to match the timing of the below case where we wait on <var>loadingPromise</var> to settle.
+      Among other things, this lets <span>prepare the script element</span> finish setting up its
+      <span>steps to run when the result is ready</span>.</p>
+     </li>
 
      <li><p>Return.</p></li>
     </ol>