[cssom] MediaList.deleteMedium can throw

Fixes #696

Author: zcorpan

cssom/Overview.bs

@@ -502,6 +502,7 @@ The <dfn method for=MediaList>deleteMedium(<var>medium</var>)</dfn> method must
  <li>If <var>m</var> is null terminate these steps.
  <li>Remove any media query from the <a>collection of media queries</a> for which
  <a lt="compare media queries">comparing</a> the media query with <var>m</var> returns true.
+ If nothing was removed, then <a>throw</a> a {{NotFoundError}} exception.
 </ol>
 
 Selectors {#selectors}