Modify the logic for double-declarative shadow root attachment

Per the discussion at whatwg/dom#1235, this PR makes the *first* declarative shadow root "win". Prior behavior was for the *last* one to remain, but this PR changes second and subsequent declarative shadow roots to report an exception instead.

source

@@ -126151,26 +126151,20 @@ document.body.appendChild(text);
      data-x="attr-template-shadowrootdelegatesfocus">shadowrootdelegatesfocus</code> attribute;
      otherwise false.</p></li>
 
-     <li><p><span data-x="concept-attach-a-shadow-root">Attach a shadow root</span> with
-     <var>declarative shadow host element</var>, <var>mode</var>, <var>clonable</var>,
-     <var>delegatesFocus</var>, and "<code data-x="">named</code>".</p></li>
+     <li><p>If <var>declarative shadow host element</var> is a <span>shadow host</span>, then
+     <span>insert an element at the adjusted insertion location</span> with
+     <var>template</var>.</p></li>
 
      <li>
-      <p>If an exception was thrown by <span data-x="concept-attach-a-shadow-root">attach a
-      shadow root</span>, then catch it, and run these steps:</p>
+      <p>Otherwise:
 
       <ol>
-       <li><p><span>Report the exception</span>.</p></li>
+       <li><p><span data-x="concept-attach-a-shadow-root">Attach a shadow root</span> with
+       <var>declarative shadow host element</var>, <var>mode</var>, <var>clonable</var>,
+       <var>delegatesFocus</var>, and "<code data-x="">named</code>". If an exception is thrown,
+       then catch it, <span>report the exception</span>, <span>insert an element at the adjusted
+       insertion location</span> with <var>template</var>, and return.</p></li>
 
-       <li><p><span>Insert an element at the adjusted insertion location</span> with
-       <var>template</var>.</p></li>
-      </ol>
-     </li>
-
-     <li>
-      <p>Otherwise:</p>
-
-      <ol>
        <li><p>Let <var>shadow</var> be <var>declarative shadow host element</var>'s
        <span data-x="concept-element-shadow-root">shadow root</span>.</p></li>