fleet prints error on `fleetctl list-machines` against empty cluster

Running fleetctl against etcd in-which no activity had yet happened (no machines were even running):

```
$ fleetctl list-machines
Error retrieving list of active machines: 100: Key not found (/_coreos.com) [6]
```

As soon as I start a machine the error goes away. I would expect an empty machine list, not an error.