Dry Run REST Registration

Registration rest service should be able to validate user details without further registration processing.
I assume registration request should have additional parameter (or flag), so called "dryRun":

1. If this parameter equals "true", registration rest service doesn't process further registration.
2. If this parameter equals "false", registration rest service processes further registration (in case there aren't validation errors).

In both cases rest service returns validation errors, if any.