add [[ErrorData]] slot to DOMExceptions

Author: ljharb

index.bs

@@ -14505,6 +14505,7 @@ The [=class string=] of a [=namespace object=] is the [=namespace=]'s [=identifi
 In the JavaScript binding, the [=interface prototype object=] for {{DOMException}}
 has its \[[Prototype]] [=/internal slot=] set to the intrinsic object {{%Error.prototype%}},
 as defined in the [=create an interface prototype object=] abstract operation.
+It also has an \[[ErrorData]] slot, like all built-in exceptions.
 
 Additionally, if an implementation gives native {{Error}} objects special powers or
 nonstandard properties (such as a <code>stack</code> property),