Need to catch exceptions to report them

Now that the default semantics are to throw, we need to be more explicit about catching.

dom.bs

@@ -3247,7 +3247,7 @@ run these steps:
 
    <li><p>If <var>records</var> <a for=queue>is not empty</a>, then <a spec=webidl>invoke</a>
    <var>mo</var>'s <a for=MutationObserver>callback</a> with « <var>records</var>, <var>mo</var> »,
-   and <var>mo</var>. If this throws an exception, then <a>report the exception</a>.
+   and <var>mo</var>. If this throws an exception, catch it, and <a>report the exception</a>.
   </ol>
 
  <li><p><a for=set>For each</a> <var>slot</var> of <var>signalSet</var>, <a>fire an event</a>