Messages hook - one message is also a list (ul)

messages.php

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) {
    $output .= "<div class=\"messages $type\">\n";
    if (!empty($status_heading[$type])) {
      $output .= '<h2 class="element-invisible">' . $status_heading[$type] . "</h2>\n";
    }
    $output .= " <ul>\n";
    foreach ($messages as $message) {
      $output .= '  <li>' . $message . "</li>\n";
    }
    $output .= " </ul>\n";
    $output .= "</div>\n";
  }
  return $output;
}