• Updates offeringsWithCompletionBlock: to fix a case where if the backend response was erroneous, the completion block would not be called.
• Also updates offeringsWithCompletionBlock: so that if there are no offerings in the RevenueCat dashboard, or no SKProducts could be fetched with the product identifiers registered in the RevenueCat dashboard, the method returns an error with instructions on how to fix the issues.
  https://github.com/revenuecat/purchases-ios/pulls/885