Make respondWith(r) throws against error conditions

spec/service_worker/index.html

@@ -1492,14 +1492,16 @@ <h1><code>event.respondWith(r)</code></h1>
         </li>
         <li>Set the <a href="#wait-to-respond-flag">wait to respond flag</a>.</li>
         <li>Wait until <var>r</var> settles.</li>
-        <li>If <var>r</var> rejected, then:
+        <li>If <var>r</var> rejected with an exception, then:
           <ol>
             <li>Set the <a href="#respond-with-error-flag">respond-with error flag</a>.</li>
+            <li><a href="http://dom.spec.whatwg.org/#concept-throw">Throw</a> that exception.</li>
           </ol>
         </li>
         <li>If <var>r</var> resolves to <var>response</var>, an instance of <code><a href="http://fetch.spec.whatwg.org/#response">Response</a></code> interface whose <code>type</code> attribute is "<code>opaque</code>", and the <code><a href="#context-enum">context</a></code> attribute is "<code>navigate</code>", then:
           <ol>
             <li>Set the <a href="#respond-with-error-flag">respond-with error flag</a>
+            <li><a href="http://dom.spec.whatwg.org/#concept-throw">Throw</a> a "<code><a href="http://dom.spec.whatwg.org/#networkerror">NetworkError</a></code>" exception.</li>
           </ol>
         </li>
         <li>If <var>r</var> resolves to <var>response</var>, an instance of <code><a href="http://fetch.spec.whatwg.org/#response">Response</a></code> interface, then: