Policy creation or update triggers policy validation code, which produces user-friendly errors if policy fails a validation check. The error messages are designed to help someone figure and fix out the cause of the error. These error messages do show up when using REST API. However, these are not show to the policy manager GUI user. UI should try to show the errors to the user. Note that validation can return multiple errors. Ideally the UI should separate these out and show them to user so that it is clear to the user that multiple errors happened.