The webCallback flow is called as a REST service, so any actions that happen on the client side need to be done via the HTTP response code and not any Mendix client microflow actions.
So in your case, I think you’d need to add/adjust the error handler flow to return a HTTP 302 with a Location header pointing to a new URL where the user should be sent. Look at the happy path in that flow – it also performs an HTTP 302 redirect.
The new URL could be a link to the login page, or a deep link to an error page.