Module doesn't work if site_$error is empty.

Currently we detect 403/404 errors like this (ignoring logintoboggan logic):

// Include both the path alias and normal path for matching.
$current_path = array(drupal_get_path_alias($_GET['q']));
if ($current_path != $_GET['q']) {
  $current_path[] = $_GET['q'];
}

if (in_array(variable_get("site_$error", ''), $current_path)) {
  $this->condition_met($context);
}

But if site_403 or site_404 variables aren't set, this approach can't work as you're matching an empty string against the current path. As a fallback to the path based approach, we could look at the actual headers being built by drupal http://api.drupal.org/api/drupal/includes%21common.inc/function/drupal_g....