Show error on invalid oidc issuer
If a valid issuer is not provided in the Foreman settings, then there is no way a user can understand why SSO failed.
The method `valid_issuer?` in openid_connect.rb checks if a valid issuer is present or not. But if that fails authentication
flow ends with a error saying `SSO failed` but does not specify why.
1) Provide a wrong issuer in the Foreman settings.
2) Try to login via SSO.