Fire <link rel=preload> error events only for network errors

A part of #1142. Although no browsers match this exactly, and implementers aren't sure this will be implementable, this is at least closer to reality than the previous spec, and reflects the direction everyone wants to go.

Author: hiroshige-g

source

@@ -26261,8 +26261,17 @@ document.body.appendChild(wbr);</code></pre>
      <li><p>Otherwise, call <var>preloadEntry</var>'s <span data-x="preload on response
      available">on response available</span> with <var>response</var>.</p></li>
 
-     <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
-     data-x="event-load">load</code> at <var>el</var>.</p></li>
+     <li>
+       <p>If <var>response</var> is a <span>network error</span>, <span
+       data-x="concept-event-fire">fire an event</span> named <code
+       data-x="event-error">error</code> at <var>el</var>. Otherwise, <span
+       data-x="concept-event-fire">fire an event</span> named <code data-x="event-load">load</code>
+       at <var>el</var>.</p>
+
+       <p class="XXX">The actual browsers' behavior is different from the spec here, and the
+       feasibility of changing the behavior has not yet been investigated. See <a
+       href="https://github.com/whatwg/html/issues/1142">issue #1142</a>.</p>
+     </li>
 
      <li><p><span>Unblock rendering</span> on <var>el</var>.</p></li>
     </ol>