command handler doesn't output the read and write errors

[nak3]

Current codes check the result of Read or Write like this:

  n, err := r.ReadAt(cmd.Buf[:length], int64(offset))
    if n < length {
        log.Errorln("read/read failed: unable to copy enough")
        return cmd.MediumError(), nil
    }
    if err != nil {
        log.Errorln("read/read failed: error:", err)
        return cmd.MediumError(), nil
    }

When Read() and Write() return error, they always make n < length true, and so they return before outputting the error.