Incorrect error management during SAML SSO

Bug description

ServerFaultException is not always thrown with the correct arguments, which can result in MissingResourceException when AM tries to display an error page.

How to reproduce the issue

Run the elastic SAML test, the secret store misconfiguration case should trigger the exception message in the container logs for example.

Expected behaviour

The error is handled without any further errors.

Current behaviour

An exception shows up in the container logs.