I have created a new set of permission, and I let the users see the topics, but when they click a topic, they are suppsoed to be blocked

These are the settings:

But what happens, when clicking any topic, instead of seeing a message that says that its blocked, they see a 404 error - this is really bad - they are supposed to know they dont have permission, but not to think that the site is broken!

This is what I get:

This is bad!

Another related question - how do I change this message?

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I need to say that they need membership, and a membership subscription link