code: func (cmd *command) sendError(ch io.ReadWriter, err error) error {
  	if err == nil {
  		return nil
  	}
  	cmd.log.Error(err)
  	message := err.Error()
  	bytes := make([]byte, 0, len(message)+2)
  	bytes = append(bytes, ErrByte)
  	bytes = append(bytes, message...)
  	bytes = append(bytes, []byte{'\n'}...)
  	_, writeErr := ch.Write(bytes)
  	if writeErr != nil {
  		cmd.log.Error(writeErr)
  	}
  	return trace.Wrap(err)
  }
docs: // sendError gets called during all errors during SCP transmission.
  // It writes it back to the SCP client