Issue #109 : add a category for NSError to check if the error is offline error
Issue #109 : add re-authentication process if cookie expires; avoid submitting events if user is not logged in
Issue #109 : whenever an authorization error happen, invalidate the current stored user information, and ask user to re log-in
Issue #109 : avoid re-prefetching by adding a safe-guard
Issue #109 : don't delete the account yet after authentication failed, since we may need that later