saml2: Add base class for errors.
saml2: Add base class for errors.

Introduce a new class: sspmod_saml2_Error, which represents
SAML 2 status codes.

ArtemTeleshev/php-saml2,sgomez/saml2,madmatt/saml2,simplesamlphp/saml2,sitya/saml2,newmen/saml2,cb8/saml2