Make togglePopover throw more exceptions

This PR makes sure that togglePopover will throw exceptions when it is
disconnected from the document or doesn't have a popover attribute.

Fixes whatwg#8999

source

@@ -82726,6 +82726,19 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
    <li><p>Otherwise, if <var>force</var> is not present or true, then run <span>show popover</span>
    given <span>this</span> true, and null.</p></li>
 
+   <li>
+    <p>Otherwise:</p>
+
+    <ol>
+     <li><p>Let <var>expectedToBeShowing</var> be true if <span>this</span>'s <span>popover
+     visibility state</span> is <span data-x="popover-showing-state">showing</span>, otherwise
+     false.</p></li>
+
+     <li><p>Run <span>check popover validity</span> given <var>expectedToBeShowing</var>, true, and
+     null.</p></li>
+    </ol>
+   </li>
+
    <li><p>Return true if <span>this</span>'s <span>popover visibility state</span> is <span
    data-x="popover-showing-state">showing</span>; otherwise false.</p></li>
   </ol>