Spring Security Examine SAML2 error code when assertion has invalid signature

Scenario

A SAML2 response assertion has an invalid signature.

Behavior

This commit changed the expected Saml2AuthenticationException error code in the integration tests.

We should examine whether the correct error code in this case is invalid_signature or invalid_assertion.

If the correct error code is invalid_signature then this commit may be reverted.

Comment From: jzheaux

Thanks, @eleftherias, this commit looks good. The information in the exception is now in the Saml2Exception cause.