fix: return error when there is no subscribe plan found

internal/services/auth/register_user.go

@@ -25,9 +25,14 @@ func (a *AuthSvc) RegisterUser(spec authdomain.RegisterUserReq) (string, error)
 		spec.SubscribePlanID = subsPlan.ID
 	}
 
+	_, err = a.subsPlanRepo.GetByID(spec.SubscribePlanID)
+	if err != nil {
+		return "", errors.New(err.Error())
+	}
+
 	subsId, err := a.subsRepo.Create(spec.SubscribePlanID)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 
 	hashedPwd, err := HashPassword(spec.Password)