(for others..) I notice the error array contains this 0-line, 1-severity, 2-msg, 3-array_of_childen (??).

It is missing the column# (which isn't that useful i guess). There is a lot of logic involved in generating the HTML formatted error string (that includes a bit more info). I think HTML stripping is the way to go if those extra bits are wanted - though more likely to break with future releases.

$dirty_html = '<img src="javascript:evil();" onload="evil();" />hello<img src="/s.gif">';
$config = HTMLPurifier_Config::createDefault();
$config->set('Core', 'CollectErrors', true);      // this is needed to collect errors
$purifier = new HTMLPurifier($config);
$clean_html = $purifier->purify($dirty_html);

$e = $purifier->context->get('ErrorCollector');   // grab errors
if ($e->getRaw())                                 // errors were present
{
  $str = $e->getHTMLFormatted($config);           // get errors in html format (not so good for log file)

  // --------- interesting code starts here ------------------------------------
  $str = str_replace('<li>', "\n", $str);         // replace <li>'s with newlines
  $str = preg_replace('/\<.*\>/Us', '', $str);    // remove all other html tags  U=ungreedy, s=(. equals newline too)
  $str = trim(htmlspecialchars_decode($str));     // replace %gt; with '>' etc - and trim spaces and preceeding newline

  // at this point $str is a text string suitable for logging.  You can display in an html page like this:
  echo "<pre>";
  echo htmlspecialchars($str);
  echo "</pre>";
}