When an IdP response to an SimpleSAMLphp SP does not contain a successful authentication, the SP will show an unhandled exception.

Although uncommon, it is a valid scenario.

Ideally the SP would handle this more graceful, with a user friendly message that describes better what is going on.

If the flow stops at the SP with an unhandled exception, the user will perceive this as a problem with the SP, while the real problem lies at the IdP.