Prefix for error messages

I need to add 'Error:' prefix to every error message (not prefix on the field name but on the whole message).

I do it like this:

function theme_status_messages($variables) {
  $display = $variables ['display'];
  $output = '';

  $status_heading = array(
    'status' => t('Status message'),
    'error' => t('Error message'),
    'warning' => t('Warning message'),
  );
  foreach (drupal_get_messages($display) as $type => $messages) {
    $myprefix = $type == 'error' ? 'ERROR: ' : '';
    $output .= "<div class=\"messages $type\">\n";
    if (!empty($status_heading [$type])) {
      $output .= '<h2 class="element-invisible">' . $status_heading [$type] . "</h2>\n";
    }
    if (count($messages) > 1) {
      $output .= " <ul>\n";
      foreach ($messages as $message) {
        $output .= '  <li>' . $myprefix . $message . "</li>\n";
      }
      $output .= " </ul>\n";
    }
    else {
      $output .= $myprefix . reset($messages);
    }
    $output .= "</div>\n";
  }
  return $output;
}

but this is only working with Drupal default messages.

Can someone help me with this?