etcdmain: better logging for discovery error

Fix #1862 

Logging as 
```
2014/12/11 15:42:26 no data-dir provided, using default data-dir ./infra1.etcd
2014/12/11 15:42:26 etcd: listening for peers on http://127.0.0.1:7001
2014/12/11 15:42:26 etcd: listening for client requests on http://127.0.0.1:4001
2014/12/11 15:42:26 etcd: stopping listening for client requests on http://127.0.0.1:4001
2014/12/11 15:42:26 etcd: stopping listening for peers on http://127.0.0.1:7001
2014/12/11 15:42:26 etcd: member infra1 has already registered with discovery service (https://discovery.etcd.io/b5d065d6e8078f60374c83a498e3d8f6), but the data-dir (infra1.etcd) on disk cannot be found.
```

instead of 

```
no data-dir provided, using default data-dir ./4e5fdea7886a46a69302ffb826351eab.etcd
etcd: listening for peers on http://0.0.0.0:2379
etcd: listening for client requests on http://0.0.0.0:2380
etcd: stopping listening for client requests on http://0.0.0.0:2380
etcd: stopping listening for peers on http://0.0.0.0:2379
etcd: client: key already exists
```

/cc @robszumski