[iOS] Handle Apple NSUnderlyingError instead of failing on GeneralError when Terms & Conditions have changed

[RobinS-S]

For iOS only: Handle Apple NSUnderlyingError instead of failing on GeneralError when Terms & Conditions have changed

This message is shown when you enable 'interrupt purchases' in the Sandbox users on App Store Connect. When placing the purchase, PurchaseAsync is called and it returns a GeneralError. What actually is happening is Apple is refusing the payment to be made until the user accepts the updated Terms of Service.

To reproduce, please visit App Store Connect and navigate to the 'Users and Access' section. In the side menu, click Testers under Sandbox. Click or create a user and tick the box 'Interrupt purchases'. Log in and out of your Sandbox account from the iOS device and try to make a purchase. After pressing the blue 'Buy' button you will get this popup.

Fixes issue where the plugin returned invalid status for the payment. An app I submitted was denied for having shown an error message for this.

Changes Proposed in this pull request:

• In case of SKError.Unknown, check for underlying NSError and check this with number 3038. There does not seem to be a constant within the Apple SDK for this. I found this by debugging and a thread on Apple's forums

Please tell me if I can make any corrections to my change or other ways of handling this are known.

A report on Apple forums: https://developer.apple.com/forums/thread/674081

[jamesmontemagno]

Should be in 4.2.0-beta

We actually could probably move it down a little bit....