Remove ErrorEventInit's error default

This ensures that ErrorEvent has equivalent expressiveness to reportError().

Fixes #7853.

source

@@ -94923,7 +94923,7 @@ dictionary <dfn dictionary>ErrorEventInit</dfn> : <span>EventInit</span> {
   USVString filename = "";
   unsigned long lineno = 0;
   unsigned long colno = 0;
-  any error = null;
+  any error;
 };</code></pre>
 
   <p>The <dfn attribute for="ErrorEvent"><code data-x="dom-ErrorEvent-message">message</code></dfn>
@@ -94943,8 +94943,9 @@ dictionary <dfn dictionary>ErrorEventInit</dfn> : <span>EventInit</span> {
   error occurred in the script.</p>
 
   <p>The <dfn attribute for="ErrorEvent"><code data-x="dom-ErrorEvent-error">error</code></dfn>
-  attribute must return the value it was initialized to. Where appropriate, it is set to the object
-  representing the error (e.g., the exception object in the case of an uncaught DOM exception).</p>
+  attribute must return the value it was initialized to. It must initially be initialized to
+  undefined. Where appropriate, it is set to the object representing the error (e.g., the exception
+  object in the case of an uncaught exception).</p>
 
   </div>