Execute a command with 'command', check the result code and throw an appropriate
        FatalError if it fails.

        Returns the "result" of a successful command.

def check_command(self, op_description, op=None, data=b'', chk=0, timeout=DEFAULT_TIMEOUT):
        """
        Execute a command with 'command', check the result code and throw an appropriate
        FatalError if it fails.

        Returns the "result" of a successful command.
        """
        val, data = self.command(op, data, chk, timeout=timeout)

        # things are a bit weird here, bear with us

        # the status bytes are the last 2/4 bytes in the data (depending on chip)
        if len(data) < self.STATUS_BYTES_LENGTH:
            raise FatalError("Failed to %s. Only got %d byte status response." % (op_description, len(data)))
        status_bytes = data[-self.STATUS_BYTES_LENGTH:]
        # we only care if the first one is non-zero. If it is, the second byte is a reason.
        if byte(status_bytes, 0) != 0:
            raise FatalError.WithResult('Failed to %s' % op_description, status_bytes)

        # if we had more data than just the status bytes, return it as the result
        # (this is used by the md5sum command, maybe other commands?)
        if len(data) > self.STATUS_BYTES_LENGTH:
            return data[:-self.STATUS_BYTES_LENGTH]
        else:  # otherwise, just return the 'val' field which comes from the reply header (this is used by read_reg)
            return val