Tells the URL session that the session has been invalidated.
The session object that was invalidated.
The error that caused invalidation, or nil if the invalidation was explicit.
If you invalidate a session by calling its finishTasksAndInvalidate() method, the session waits until after the final task in the session finishes or fails before calling this delegate method. If you call the invalidateAndCancel() method, the session calls this delegate method immediately.