Don't cache HTTP errors in the import module cache

Multiple parallel imports will still only fetch once, and if the fetch
fails, will both error. A subsequent import (dynamic import, a
dynamically inserted via `<script src="..." type="module">`, or a static
import from either of these) to the same specifier will result in a
re-fetch if the previous fetch failed.

source

@@ -106886,14 +106886,20 @@ document.querySelector("button").addEventListener("click", bound);
    <li><p>Let <var>moduleMap</var> be <var>settingsObject</var>'s <span
    data-x="concept-settings-object-module-map">module map</span>.</p></li>
 
-   <li><p>If <var>moduleMap</var>[(<var>url</var>, <var>moduleType</var>)] is
-   "<code data-x="">fetching</code>", wait <span>in parallel</span> until that entry's value
-   changes, then <span>queue a task</span> on the <span>networking task source</span> to proceed
-   with running the following steps.</p></li>
+   <li>
+    <p>If <var>moduleMap</var>[(<var>url</var>, <var>moduleType</var>)] is
+    "<code data-x="">fetching</code>", wait <span>in parallel</span> until that entry's value
+    changes, then <span>queue a task</span> on the <span>networking task source</span> to run these
+    steps:</p>
+
+    <ol>
+     <li><p>If <var>moduleMap</var>[(<var>url</var>, <var>moduleType</var>)] <span
+     data-x="map exists">exists</span>, run <var>onComplete</var> given
+     <var>moduleMap</var>[(<var>url</var>, <var>moduleType</var>)], and return.</p></li>
 
-   <li><p>If <var>moduleMap</var>[(<var>url</var>, <var>moduleType</var>)] <span
-   data-x="map exists">exists</span>, run <var>onComplete</var> given
-   <var>moduleMap</var>[(<var>url</var>, <var>moduleType</var>)], and return.</p></li>
+     <li><p>Run <var>onComplete</var> given null, and return.</p></li>
+    </ol>
+   </li>
 
    <li><p><span data-x="map set">Set</span> <var>moduleMap</var>[(<var>url</var>,
    <var>moduleType</var>)] to "<code data-x="">fetching</code>".</p></li>
@@ -106950,8 +106956,8 @@ document.querySelector("button").addEventListener("click", bound);
        <span>ok status</span>,</p></li>
       </ul>
 
-      <p>then <span data-x="map set">set</span> <var>moduleMap</var>[(<var>url</var>,
-      <var>moduleType</var>)] to null, run <var>onComplete</var> given null, and abort these
+      <p>then <span data-x="map remove">remove</span> <var>moduleMap</var>[(<var>url</var>,
+      <var>moduleType</var>)], run <var>onComplete</var> given null, and abort these
       steps.</p>
      </li>
 
@@ -143816,6 +143822,7 @@ INSERT INTERFACES HERE
   Logan<!-- on moz irc -->,
   Logan Moore,
   Loune,
+  Luca Casonato,
   Lucas Gadani,
   &#x0141;ukasz Pilorz,
   Luke Kenneth Casson Leighton,