For example, within UserActions.php are these two error messages, which are then stored in an array:
$flash_error .= sprintf(_m('The specified e-mail is already used by %s') , $user_email['s_username']) . PHP_EOL;
$error[] = 3;
$flash_error .= _m('The name cannot be empty').PHP_EOL;
$error[] = 10;
Is there a list of errors to follow in order to implement error messages?