More strict check for topic of all unsubscribe-related operations

[qiuzhong]

All the unsubscribe-related operations: unsubscribe and unsubscribe_service should be treated as more strict check, like type checking or whether the topic exists.

Here is the current implementation of unsubscribe operation.

    this._registerOpMap('unsubscribe', (command) => {
      debug(`unsubscribe a topic named ${command.topic}`);
      this._resourceProvider.destroySubscription(command.topic);
    });

This is simply hands the control to the resourceProvide during unsubscribe and it only handles the normal branch: when this topic exists.

  destroySubscription(topicName, bridgeId) {
    if (this._subscripions.has(topicName)) {
      let handle = this._subscripions.get(topicName);
      if (handle.hasCallbackForId(bridgeId)) {
        handle.removeCallback(bridgeId);
        handle.release();
        if (handle.count === 0) {
          this._subscripions.delete(topicName);
        }
      }
    }
  }

As a result, invalid type of topic brings a none level response, which means the unsubscribe succeed. This is wrong and confusing.

What we expected:

• Invalid topic type: a error returned.
• Valid topic type but not existed, a warning returned.

For the unsubscribe_service operation, same proposal had been issued in the comments of PR #82.

[qiuzhong]

Verified as PR #89 comments.