Implemented error handling for XBlock instances

Failed XBlock loads previously logged a warning that went nowhere. They are now stored in a list of failed XBlock instances (with their exception). @pmitros @nedbat 
Because `load_classes` no longer actually _loads_ classes (that has been moved to `_load_entry_points`), this method doesn't really do much — just returns the attribute `successful_loads`. The name should remain `load_classes` for the sake of compatibility with pretty much anything that uses XBlock.