[fix][#33893] W3C error validation for template isis on error page. Fixes #3818

See http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33893&start=0

Update index.php

see : http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33893&start=0