Allow errors to be handled via callback instead of redirecting to fixed URL based on config

[kevinvrenewi]

Goals

We need to implement localization support in our app. For a proper customer experience, it must be possible to redirect a user to a localized login page as well as any error pages.

Background

We have a localized app in various languages. Whenever an error occurs in the callback from the IdP, a redirect to a fixed error page is being done. This URL is not localized which is confusing for our end users.

Proposal

Allow custom callback for error handling so we can write a cookie with the locale when redirecting a user away and read out this cookie whenever an error occurs in order to redirect the user to the proper localized error page.