Work around theme_status_messages() limitation to display checkout form errors outside panes

If there is one error outside the checkout panes, it is not displayed. The problem is this line:

$previous_messages['error'] = array_diff($previous_messages['error'], $errors[$pane_id]);

It can return an array not starting from 0:

var_dump(array_diff(array('a','b'), array('a')));
array(1) {
  [1]=>
  string(1) "b"
}

But theme_status_messages expects that the index should start from 0, at least when there is one element. While this could be considered a bug in Drupal core, it's easier to fix in Commerce.