Improve SAML debuggability
Created by: beyang
Make INSECURE_SAML_LOG_TRACES=1
also write underlying errors in HTTP responses (visible to end-users). Currently, it just causes additional information to be printed to frontend
stdout.
This targets the debugging situation where the Sourcegraph site admin may be a different individual than the person who configures the SAML IdP. This will help us cut down on back-and-forth in email debugging chains.