In config.local.php
Add this code directly underneath the copyright notice:
if ($_SERVER['REMOTE_ADDR'] == '000.000.000.000') {
define('DEVELOPMENT', true);
error_reporting(E_ALL);
ini_set('display_errors', 'On');
}
Where
000.000.000.000
equals your IP address.
This will allow you to review any error messages when displayed, and not to your customers.