cmd/livepeer: fix "Errorf call has arguments but no formatting directives" (#957)
cmd/livepeer: fix "Errorf call has arguments but no formatting directives" (#957)

Fix build warning:
```
# github.com/livepeer/go-livepeer/cmd/livepeer
cmd/livepeer/livepeer.go:199:14: Errorf call has arguments but no formatting directives
```
livepeer/go-livepeer,livepeer/go-livepeer