Update Single Pages dashboard controller to pass error codes

Update Single Pages dashboard controller to pass error codes rather than error messages in the URL.

There are still 2 places in this controller that will pass the entire error message, but I'm not entirely sure how to go about fixing them. The below code appears twice in this controller:

> $this->redirect('/dashboard/pages/single', $this->token->getErrorMessage(), 1);

Is there a preferred way to pass these dynamic error messages? As a querystring, through a session flash message, etc?