make getProductsForIds and purchase handle errors in the product requests

This pull request makes adds a new getProductsForIds:completion:error: method that takes care of any errors that may happen during the product requests. 

The signature without error handling is still left, and passes `nil` to the error handler above.

I also bumped the Podfile version, if you want to add a new version to CocoaPods that'd be great. Thanks!