Attach timing info and URL to network errors

Set an opaque timing info and the original request URL to the error
resposne, and call the processResponseDone callback to allow the caller
to report it. This should already be the case for reporting network
errors in the fetch() API.

Closes whatwg#1215

fetch.bs

@@ -2015,6 +2015,7 @@ known as an <dfn export id=concept-aborted-network-error>aborted network error</
 <p>A <a>network error</a> is a <a for=/>response</a> whose
 <a for=response>status</a> is always 0,
 <a for=response>status message</a> is always the empty byte sequence,
+<a for=response>timing allow passed flag</a> is always unset,
 <a for=response>header list</a> is always empty, and
 <a for=response>body</a> is always null.
 
@@ -4103,6 +4104,19 @@ steps:
    <var>processBody</var>, <var>processBodyError</var>, and <var>fetchParams</var>'s
    <a for="fetch params">task destination</a>.
   </ol>
+
+ <li><p>If <var>fetchParams</var>'s <a for="fetch params">process response done</a> is non-null, and
+ <var>response</var> is a <a>network error</a>, then:
+
+ <ol>
+  <li><p>Set <var>response</var>'s <a for=response>URL list</a> to a <a for=/>list</a> containing
+  <var>fetchParams</var>'s <a for="fetch params">request</a>'s <a for=request>URL</a>.
+
+  <li><p><a>Queue a fetch task</a> to run <var>fetchParams</var>'s
+  <a for="fetch params">process response done</a> given <var>response</var>, with
+  <var>fetchParams</var>'s <a for="fetch params">task destination</a>.
+ </ol>
+
 </ol>
 
 <p>To <dfn>finalize response</dfn> given a <a for=/>fetch params</a> <var>fetchParams</var> and a