Use of bang tag by non-member of group should trigger a user alert

If a user is not a member of a group but they post a message with a bangtag like !group, the group name will not be autolinked and the group will not receive it, but also, the user doesn't get any notification that that is why the autolink didn't happen. It just looks like it failed and they don't know why.

It seems right to not do the autolink (avoid spam from nonsubscribers), but a message to notify the user that the reason is because they're not actually subscribed to the group, would be very useful. It's very mystifying, when you think you ARE subscribed to a group and for whatever reason are not.