Password must contain at least 6 characters and one uppercase letter.

As the password is stored as a hash in a database, I added validation before the user is created.
Still we have `SignupError` here, but I guess that `ValidationError` is a proper class. Perhaps because of that, when I try to change the password and violate our constraint,  the message“The old password you provided is incorrect, your password was not changed.” is displayed.