[IMP] partner: find default avatar based on gravatar

Task: https://www.odoo.com/web?debug=#id=8137&view_type=form&model=project.task&action=333&active_id=131
Pad: https://pad.odoo.com/p/r.lVlmyH1yY7REpZvS
Do we really want an exception for this kind of feature and error ? logger.warning will be sufficient I think.