no-glib: WIP, remove g_set_error.

Most of the g_set_error's is not very useful. Now errors are logged
on-site. Only problem is that some g_set_error is used for returning
errors to the client.

To solve that, now detailed error messages are no longer returned to
the client, but logged. The client only receive an abstract error.
Actually this approach is not that different from what mpd does now.