[giow] (3) Drop the ability to catch 'error' events from <iframe>, since nobody implemented it after 5 years.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23609
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8239 340c8d12-0b0e-0410-8428-c7bf67bfef74