@Override
    public String toString() {
        return new StringJoiner(", ", AsyncResultImpl.class.getSimpleName() + "[", "]")
            .add("result=" + result)
            .add("error=" + error)
            .toString();
    }
@see io.apiman.gateway.engine.async.IAsyncResult#getError()
gateway/engine/core/src/main/java/io/apiman/gateway/engine/async/AsyncResultImpl.java