[ES6 modules] Update module linking procedures to follow latest spec updates

This CL updates Blink's impl of #internal-module-script-graph-fetching-procedure
and #fetch-the-descendants-of-and-instantiate-a-module-script to match
recent spec change: whatwg/html#2595 .

Tests: external/wpt/html/semantics/scripting-1/the-script-element/module/specifier-error.html
Bug: 727299, 594639
Change-Id: I022b3b380b408a6d5c75a59d161aea4fe2868f48
Reviewed-on: https://chromium-review.googlesource.com/528724
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478219}
WPT-Export-Revision: a1fd48a745a56045e25c3365a69f3fce0358074f

Author: nyaxt

html/semantics/scripting-1/the-script-element/module/specifier-error.html

@@ -4,6 +4,8 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
+    setup({allow_uncaught_exception: true});
+
     window.log = [];
 
     window.addEventListener("error", ev => log.push(ev.error));