Update password hash docs, add more validation on password hash

user/types.go

@@ -249,7 +249,8 @@ var (
 	ErrInvalidArgument        = errors.New("invalid argument")
 	ErrUserNotFound           = errors.New("user not found")
 	ErrUserExists             = errors.New("user already exists")
-	ErrPasswordHashInvalid    = errors.New("password hash but be a bcrypt hash, use 'ntfy user hash' to generate")
+	ErrPasswordHashInvalid    = errors.New("password hash must be a bcrypt hash, use 'ntfy user hash' to generate")
+	ErrPasswordHashWeak       = errors.New("password hash too weak, use 'ntfy user hash' to generate")
 	ErrTierNotFound           = errors.New("tier not found")
 	ErrTokenNotFound          = errors.New("token not found")
 	ErrPhoneNumberNotFound    = errors.New("phone number not found")