Page to display when using !confirm_unsubscribe_url twice

When the recipient of a newsletter clicks on the !confirm_unsubscribe_url link twice (or if the e-mail address has already been removed before the link is clicked), Simplenews performs a drupal_not_found(), telling the user the page requested doesn't exist. This confuses or even annoys users who think their unsubscription request is not honored.

The patch attached allows the admin to specify a page to be called in these cases, providing appropriate description of the situation (usually, to calm the user by saying that their address has already been removed). If not specified, it will default to the original behavior, showing the usual not found page.