Catch errors while upgrading customized built-in elements

Closes whatwg/html#5084.

Tests: web-platform-tests/wpt#23072

dom.bs

@@ -5996,8 +5996,23 @@ given a <var>document</var>, <var>localName</var>, <var>namespace</var>, and opt
    <a for=Element><code>is</code> value</a> set to <var>is</var>, and <a for=Node>node document</a> set to
    <var>document</var>.
 
-   <li><p>If the <var>synchronous custom elements flag</var> is set,
-   <a lt="upgrade an element">upgrade</a> <var>element</var> using <var>definition</var>.
+   <li>
+    <p>If the <var>synchronous custom elements flag</var> is set, then run this step while
+    catching any exceptions:
+
+    <ol>
+     <li><p><a lt="upgrade an element">Upgrade</a> <var>element</var> using <var>definition</var>.
+    </ol>
+
+    <p>If this step threw an exception, then:</p>
+
+    <ol>
+     <li><p><a>Report the exception</a>.
+
+     <li><p>Set <var>result</var>'s <a for=Element>custom element state</a> to
+     "<code>failed</code>".
+    </ol>
+   </li>
 
    <li><p>Otherwise, <a>enqueue a custom element upgrade reaction</a> given <var>result</var> and
    <var>definition</var>.