refactor errors

core/in_memory_db.go

@@ -89,9 +89,11 @@ func (db *InMemoryDb) Renew(id CustomerId) error {
 	if !exists {
 		return errors.New(fmt.Sprintf("customer %s does not exist", id))
 	}
-	setXor, attrsXor := customer.RenewKeys()
+	setXor, attrsXor, err := customer.RenewKeys()
+	if err != nil {
+		return err
+	}
 	db.Customers[id] = customer
-	var err error
 	for _, user := range db.Users {
 		if user.CustomerId == id {
 			err = user.RenewKeys(setXor, attrsXor)