Add a message to the 404 page to indicate it may be a permission issue
Zendesk: https://gitlab.zendesk.com/agent/tickets/15943
We display a 404 page when users don't have permission to a project to avoid leaking information about projects/groups. It could be advantageous to add an ambiguous message to the 404 page saying 'it either doesn't exist or you don't have permission to view the resource'. This would be similar to what we do for password reset: 'If your account exists in our system, you will receive a message'. We are descriptive but avoid disclosing whether the account exists.