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.