Interrupted Purchase not handled

## Bug Report
When the purchasing is interrupted by apple EULA updated, it fails with unknown error.

**To Reproduce**
Steps to reproduce the behavior:

1. In App Store Connect, Users and Access -> Sandbox Testers check "Interrupt Purchases for This Tester".
2. Purchase as described in the Readme.md.

**Expected behavior**
SwiftyStoreKit.purchaseProduct won't call its callback until the "You are all set" window appears.

## Platform Information
- OS: iOS 14.4
- Purchase Type: auto-renewable subscription
- Environment: sandbox
- SwiftyStoreKit version: 0.16.3

**Screenshots**