LibWeb: Empty CE reaction queue instead of destroying it on exception

If an exception occurs in a custom element constructor, we clear the
reaction queue by destroying it, instead of emptying the Vector.
3da6916383/Userland/Libraries/LibWeb/DOM/Element.cpp (L2033)

This causes a UAF here, as async upgrades (i.e. custom elements not
created by document.createElement) are performed in this loop:
3da6916383/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp (L657)

Fixes crash when loading https://github.com/SerenityOS/serenity

Tests/LibWeb/Text/expected/HTML/custom-elements-throw-in-constructor.txt

@@ -0,0 +1,2 @@
+   Entered TestElement constructor, throwing.
+PASS! (Didn't crash)