I have a Sitecore 9 site configured to use an OWIN provider as described in this article. The site also allows users to authenticate via forms authentication. The two authentication methods are working at this point.
Here's the issue: Whenever a user hits a page that they are unauthorized to view, Sitecore immediately redirects them to the OWIN providers's login page. What we need instead is for Sitecore to display a custom 403 Unauthorized page with relevant information. We have installed the plugin "Sitecore Error Manager", so we already have the ability to display a 403 page.
How do we change this behavior and tell Sitecore to redirect to our 403 page instead of the federated auth login page?