Add rule and alert for SAML login errors

This MR adds alert for detecting SAML login request errors (500 errors). We recently had an incident where the error rate for a particular controller increased due to an application error (inefficient DB query). To quickly detect similar problems in the future, we'd like to add an alert for it.

• Related incident: gitlab-com/gl-infra/production#17856 (closed)
• Incident review: gitlab-com/gl-infra/production#17863 (closed)