Improve handling of python path setting errors

[jiajic]

The current implementation with GiottoClass::set_giotto_python_path() quietly handles a default python path search + connection attempt, returning only a NULL if it all fails.
This prevents Giotto from being overly chatty when initiallization is called in settings where python is not being used, but masks actual errors and makes them hard to debug.

See: drieslab/Giotto#1104