overlayfs: more helpful output when not supported

based on https://github.com/docker/docker/pull/7619#discussion_r20385086

```
INFO[0000] +job serveapi(unix:///var/run/docker.sock)
INFO[0000] Listening for HTTP on unix (/var/run/docker.sock)
ERRO[0000] 'overlayfs' not found as a supported filesystem
FATA[0000] driver not supported
```

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>
ah. english is hard.