How to throw error to user when saving post

I made a custom post type and on saving its additional data I want to check if I a published post exists by its name. It works alright if there is, but I would like to throw some notice if article is not found.

    $post_exists = $wpdb->get_row("SELECT post_name FROM wp_posts WHERE post_name = '" . $_POST['article_name'] . "' AND post_type = 'post'", 'ARRAY_A');
    if($post_exists)
        update_post_meta($id, 'article_name', strip_tags($_POST['article_name']));
    else
        ???

I noticed there is http://codex.wordpress.org/Class_Reference/WP_Error but I dont think that is what I want since debugging is set to false?. Error could be anything - some usual notice would be fine, but even simple javascript alert could be good. Currently it tells me that the post is saved with green lights which doesnt seem right