unique-title-checker-only-unique-error.php

<?php
/**
 * Unique Title Checker Errors Only
 *
 * @package     unique-title-checker
 * @author      Bernhard Kau
 * @license     GPLv3
 *
 * @wordpress-plugin
 * Plugin Name: Unique Title Checker Errors Only
 * Plugin URI: https://gist.github.com/2ndkauboy/140116e47f2d6c8ae25b002592ac45eb
 * Description: Only show the error message for non-unique titles, but not success messages
 * Version: 1.0.0
 * Author: Bernhard Kau
 * Author URI: http://kau-boys.de
 * Text Domain: unique-title-checker
 * License: GPLv3
 * License URI: http://www.gnu.org/licenses/gpl-3.0
 */

add_filter( 'unique_title_checker_only_unique_error', '__return_true' );