Raising error response for invalid identity

What does this MR do and why?

This solves regression bug raised here - #385377 (closed).

The issue happened due to refactoring of logic under MR - !105378 (merged)

Screenshots or screen recordings

Sentry Report - https://sentry.gitlab.net/gitlab/gitlabcom/issues/4065492/?referrer=gitlab_plugin

Error happens here - https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/api/scim.rb#L177 due to https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/scim/group_saml_provisioning_service.rb returning nil as result

How to set up and validate locally

ProvisioningService was returning nil for case there is an error in saving an invalid identity. I have fixed the bug by returning error_response in case of error and have added the respective test case