Better distinguish I/O Error and I/O timeout, and equipe sendData with a return value

2 relatively related feature requests:

1)  In `AR488_GPIBbus.h`, there is

```C
  RECEIVE_ERR       // Receive timeout or error
```

It would be nice if a distinction was made between timeout and error.

2) `sendData` has no return value. Also there, a distinction between 'OK', 'ERROR' and 'TIMEOUT' would be really helpful.

