LibWeb: Report exceptions from custom element upgrades to global object

Author: https://github.com/ADKaster
Commit: 5be4825504
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2543
Reviewed-by: https://github.com/tcl3 ✅

Tests/LibWeb/Text/input/HTML/custom-elements-throw-in-constructor.html

@@ -2,6 +2,8 @@
 <script src="../include.js"></script>
 <script>
     test(() => {
+        removeTestErrorHandler()
+        window.addEventListener("error", (event) => { println(`${event.message}`); })
         class TestElement extends HTMLElement {
             constructor() {
                 super();