How to edit the Access Denied message?

Does anyone know how to edit this message?:

Access Denied
You do not have permissions to perform the requested action.

I wanted to add some additional actions for the user on how to get help when hitting this message. The page I'm getting to on testing is when trying to access a group that the user doesn't have permissions for, but the URL does not change to a custom page (so I can't access it via the Theme editor) and I don't see anything through the Core Message widget in Widget Studio, or anywhere else in the Administration menu that I can think of to modify this text. Core messages is the only thing I can think of that makes sense, especially when inspecting the page, but I can't find any options:

Is this possible? Thanks in advance!