c_extension.c: initiate a Python exeptions in case the camera has been disconnected:

If for any reason (e.g. the camera has been disconnected) the data stream from the camera is no longer available, without these checks the entire python interpreter crashes with a nice segmentation fault.

These allow to handle the error from within Python with the following code snippet:

```
    try:
        o3000.video_images_get()
    except RuntimeError:
        # handle the error, e.g. try to restart the o3000 driver
```