Python bindings: Enhance error reporting
Python bindings: Enhance error reporting

For the C functions returning a NULL pointer on error, the Python
bindings will now print an error message relative to the value of
the 'errno' variable.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio