Feature Request: Add an error callback to CheckSessionIFrame

[EmperorArthur]

I have encountered at least one custom OIDC server which returns "error" if a user is not logged in.

Unfortunately, the only ways to do something with this information is to use hacks or bypass "CheckSessionIFrame" completely. I would like to add an onError callback or similar to be able to handle this situation. It may also be useful in other scenarios as well.