Show proper error in SCIM create user endpoint

What does this MR do?

When SCIM create user endpoint is called, but user data is not valid and the user is not saved, we are calling member method in provisioning_service.rb which tries to create a group member for no valid user. If this particular group is using allowed domain whitelisting, this validation is throwing an error due to the lack of provided email. This MR makes sure there is no attempt of creating group member if user is not saved due to the validations not passing.

closes #13997 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team