LibWeb: Implement ErrorEvent

This will be used by the new EventTarget to check if it needs to do
special error event handling. Currently it isn't used for anything
else.

Userland/Libraries/LibWeb/Forward.h

@@ -128,6 +128,7 @@ class CanvasRenderingContext2D;
 class CloseEvent;
 class DOMParser;
 class DOMStringMap;
+class ErrorEvent;
 struct EventHandler;
 class EventLoop;
 class HTMLAnchorElement;
@@ -330,6 +331,7 @@ class DOMRectWrapper;
 class DOMStringMapWrapper;
 class DOMTokenListWrapper;
 class ElementWrapper;
+class ErrorEventWrapper;
 class EventListenerWrapper;
 class EventTargetWrapper;
 class EventWrapper;