Better error message when local user conflicts with SAML user

[klizhentas]

Description

Currently when local user exists with the same name as SAML/OIDC user and SAML user tries to login, user gets error "Failed to process SAML callback" which is not helpful, also no error in the logs.

Return more helpful error in this case.

[kontsevoy]

Note: because it's a login event, this error must be logged into the audit log (it's ok to duplicate those with stderr). usually we're pretty good at this, just making sure it continues. the standard/recommended procedure for troubleshooting logins should be watching teleport's audit instaead of re-starting teleport with -d