mqtt: No error if topic subscription fails

[jonnor]

Can for instance happen if there are permission problems.

[jonnor]

Related, there is no error when sending fails due to permissions problems. However in this case, the broker (at least on Moquitto 1.3.4) does not disconnect the client, or seem to give any other indication of something wrong, so it seems uncatchable.
This is why I ended up checking if data did come out on the topic when testing permissions setup in a deployment, https://github.com/bitraf/bitraf-iot/blob/master/test/access.coffee