@Override
    public String toString() {
        StringBuilder m = new StringBuilder("Ping result");
        activeDocuments().ifPresent(docCount -> m.append(" active docs: ").append(docCount));
        targetActiveDocuments().ifPresent(docCount -> m.append(" target active docs: ").append(docCount));
        if (isBlockingWrites)
            m.append(" blocking writes: true");
        error().ifPresent(e -> m.append(" error: ").append(error));
        return m.toString();
    }
Returns a string which included the ping info (if any) and any errors added to this
container-search/src/main/java/com/yahoo/prelude/Pong.java