Subscription confirmations for subscriptions via embedded profile

If you are already subscribed to a mailing list group and attempt to re-subscribe via the subscription page, the form throws an error (see CRM-2636).

However, if mailing lists subscription is exposed via an embedded profile (e.g. in event registration), a subscription confirmation email will be sent even if you are already a member of the relevant group(s).

In this case, it would be nice if the form did NOT throw an error (i.e. the registration goes through), but an informational message was added to the confirmation screen.