fix subscription hanging on error and add a stop subscription option inside the callback

[hgiasac]

• Fix the deadlock issue that the subscription client can't receive error events from the channel.
• Close the connection when there is no running subscription
• add a particular error that stops the subscription inside the callback

subscriptionId, err := client.Subscribe(&query, nil, func(dataValue *json.RawMessage, errValue error) error {
    // ...
    // return this error to stop the subscription in the callback
    return ErrSubscriptionStopped
})