Use "NotSupportedError" when trying to attachShadow() twice

Discussed in #760. This makes the various cases less distinguishable,
which is desirable. With this change, all three cases behave the same
between:

- A custom element with disabled-shadow
- A custom element with a shadow tree
- A built-in element on the blocklist

dom.bs

@@ -6768,7 +6768,7 @@ invoked, must run these steps:
  "<code>span</code>", then <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
 
  <li><p>If <a>context object</a> is a <a for=Element>shadow host</a>, then <a>throw</a> an
- "{{InvalidStateError!!exception}}" {{DOMException}}.
+ "{{NotSupportedError!!exception}}" {{DOMException}}.
 
  <li><p>Let <var>shadow</var> be a new <a for=/>shadow root</a> whose <a for=Node>node document</a>
  is <a>context object</a>'s <a for=Node>node document</a>, <a for=DocumentFragment>host</a> is