Properly handle private datasets when trying to access them #324

@minoriinoue got a very unhelpful message in issue #308 which is caused by the dataset associated with the task being private. We need to figure out a way how to handle those in python. I suggest raising a PrivateDatasetError.

@joaquinvanschoren shouldn't the task be private if the dataset is private in the first place?

Script to reproduce:

import openml

openml.tasks.get_task(103)