Investigate 503 and 500 errors on Groups::SsoController#sign_up

What

Investigate 500 errors and 503 errors from kibana logs and sentry.

The 503 errors could have been due to a deploy, but the raise_unless_request_is_gitlab_initiated! 500 errors could be due to an unusual user flow or trying to start sign ups from the identity provider. In these cases we should probably present a more friendly outcome.

Steps to investigate

• Try initiating SAML sign in from identity provider with group managed off
• Try initiating SAML sign up from identity provider with group managed on
• Try and find flows that redirect back to the identity provider that might clear Gitlab::Auth::Saml::OriginValidator::AUTH_REQUEST_SESSION_KEY
• Check for paths that might end up in a 503 error