Sorry, I didn’t say I would add it or would do it in this version. Keep making suggestions. I’ll add them if I think it adds some value to the resource.
Regarding your request, this is the ERROR CODE and it is used for support to identify where exactly the error happened, which consists the app + the place it happened (controller) + / specific place in the code, so obviously it can’t be customizable. That will not change.
You can change the message itself in your ACP: “You do not have permission to view this forum”.