log sink read errors are not logged

[John A Meinel]

We are seeing log sink read disconnect messages on Prodstack. However, bumping logging to a higher level isn't useful for debugging because we print:

  logger.Debugf("disconnected, %p", socket)

So we declare that it was disconnected but we just print the hex socket identifier, which isn't useful. We need to log the error.

[John A Meinel (jameinel)]

https://github.com/juju/juju/pull/10914