MDL-56899 filter: suppress dom parsing errors

Domdocument does not know about <video> and <audio> tags, also it's too strict for html style
solution from http://stackoverflow.com/questions/6090667/php-domdocument-errors-warnings-on-html5-tags

lib/weblib.php

@@ -1321,7 +1321,9 @@ function format_text($text, $format = FORMAT_MOODLE, $options = null, $courseidd
 
     if ($options['blanktarget']) {
         $domdoc = new DOMDocument();
+        libxml_use_internal_errors(true);
         $domdoc->loadHTML('<?xml version="1.0" encoding="UTF-8" ?>' . $text);
+        libxml_clear_errors();
         foreach ($domdoc->getElementsByTagName('a') as $link) {
             if ($link->hasAttribute('target') && strpos($link->getAttribute('target'), '_blank') === false) {
                 continue;