Add error message when iio crash in python

[gardiens]

Hello,

When iio is loaded and a failure occurs in Python, the only feedback currently produced is a long, opaque string, and the script aborts with a generic "NULL pointer access" error. This message provides little diagnostic value.

To improve error reporting, I modified the read interface so that the last IIO error is retrieved and surfaced as a standard Python ValueError containing the original error message.

Example:

img = iio.read(path)

File ~/.local/lib/python3.8/site-packages/iio.py:72, in read(filename)
     err = __iio_get_last_error()
     if err:
         raise ValueError(err.decode("utf-8"))
     raise ValueError("iio_read returned NULL without error message")

Resulting in:

ValueError: compressed scanlines not implemented yet
This provides a clearer and more informative error message for users.