Unfortunately that is something you will need to add as a custom feature. I don't check that at all. My kit has a hand full of error messages (like checking if an email address is validly formatted or that the password matches the verification) but apart from that it only relays any errors that WordPress or your site sends back. In the case of username and email, that is part of the registration process and I just say: "Do registration(); If there is a problem, send back what it was". WordPress determines if anything is wrong with the supplied info and if not I just relay to you what it said.
WordPress does have functions that you can use for stuff like that but I didn't implement any functions check for that, sorry.