Distinguish Validation Exceptions in Rails 3
begin
  new_user = User.new
  new_user.name = new_user_name
  new_user.email = new_user_email
  new_user.password = new_user_password
  new_user.password_confirmation = new_user_password
  new_user.save!
rescue PasswordTooShortException
   render :status => 452 and return
rescue EmailAlreadyTakenException
   render :status => 453 and return
end