[Django]-Django: what kind of exceptions does create_user throw?

If you are not using Django Auth Forms then you have to do the validation manually. The simple way is when user submit the custom form you should check:

• if username is valid, match with the regex e.g. \w+
• check if username is already taken or not e.g. User.objects.filter(username=username).exists()
• check if email is already used for an account e.g. User.objects.filter(email=email).exists()
• then create the user

FYI: there are some already Django Auth packages e.g. django-allauth, django-userena and many more which handles all the stuff pretty well, instead of reinventing the wheel by your self.