bugfix, return err when readConf() failed

[wankai]

an obviously bug in readConf() function

[philips]

lgtm. @xiangli-cmu @unihorn ?

[yichengq]

I guess it might want to filter out io.ErrNotExist.
@xiangli-cmu

[wankai]

@unihorn , config is as follow. if config file doesn't exist, peers will be nil, then this raft server will have no idea of others.

type Config struct {
    CommitIndex uint64 `json:"commitIndex"`
    // TODO decide what we need to store in peer struct
    Peers []*Peer `json:"peers"`
}

[yichengq]

@wankai It may be a new raft machine, and doesn't have that file yet.