Spring Security Saml2Error should be in a core package

Saml2Error and Saml2ErrorCodes are currently in org.springframework.security.saml2.provider.service.authentication; however, they are not specific to authentication.

Moving them to org.springframework.security.saml2.core has the benefit of being structured similiarly to OAuth2Error and OAuth2ErrorCodes.

Further, as other classes need error support, moving this up a package will alleviate package tangles.