Do not loose user data from user signup page if username already exists

General: never ever throw away any information the user has entered.

When a user tried to make a new account, don't clear the other fields when one is invalid.

A user said: "I think you should check for username availability before making users enter all of their other information. I just tried to create the username “nick” entered all my info (email, pw twice etc) and when I tried to register all my info was lost and I was given the error “Account already exists”. I was surprised that nick was available, but figured if it wasn’t that I would have been alerted before moving to the next field.