zlf: panic when cannot connect target redis

```
[+] opening logs: /var/run/log.sock
[+] connecting redis: 10.4.0.250:6379
panic: dial tcp 10.4.0.250:6379: connect: connection refused

goroutine 1 [running]:
main.main()
	/opt/go/src/github.com/threefoldtech/zosv2/cmds/zlf/main.go:55 +0x79e
```