`BackendErrorTests`: added explicit test for subscriber not found errors (#1481)
`BackendErrorTests`: added explicit test for subscriber not found errors (#1481)

The race condition fixed #1479 is luckily not a big deal because posting the attributes would have been retried.
This test ensures that behavior in a concise way.
RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios