public String toString() {
            switch(code) {
            case SUCCESS:
                return "OK: " + message;
            case ABORTED:
                return "ABORTED: " + message;
            case FAILURE:
                return "FAILURE: " + message;
            case TIMEOUT:
                return "TIMEOUT: " + message;
            }

            return "Unknown error";
        }
The result of the visitor, containing a completion code and an optional
error message.
documentapi/src/main/java/com/yahoo/documentapi/VisitorControlHandler.java