code: public String getErrorMessage()
  	{
  		if (!isNegativeResponse())
  			return "no error";
  		final int err = data[0] & 0xFF;
  		if (err > errors.length - 1)
  			return "unknown error code";
  		return errors[err];
  	}
docs: Returns a descriptive error message on a negative response, as determined by
  {@link #isNegativeResponse()}.
  <p>
  A negative response contains an error information code, which is used to find the
  associated message.<br>
  If invoked on positive response, "no error" will be returned.
  
  @return error status message as string