Handle missing or modified RelayState during SAML authentication

[jmpesp]

If a SAML login is IDP initiated (or SP initiated but the IDP eats our RelayState and supplies their own), then we can't expect RelayState to be properly set, but our current code in login_saml returns a 500 if it doesn't match our format:

        let relay_state =
            if let Some(value) = relay_state_string {
                Some(RelayState::from_encoded(value).map_err(|e| {
                    HttpError::for_internal_error(format!("{}", e))
                })?)
            } else {
                None
            };

We should instead probably continue with the rest of the function, as it's a bad user experience for them to see a 500 when the log in from their IDP.