[in_app_purchase] Expose in_app_purchase_exception.dart correctly (flutter#3874)

* Expose in_app_purchase_exception.dart correctly

* Update iOS implementation to use new instance

Author: mvanbeusekom

packages/in_app_purchase/in_app_purchase_ios/lib/src/in_app_purchase_ios_platform.dart

@@ -48,7 +48,7 @@ class InAppPurchaseIosPlatform extends InAppPurchasePlatform {
 
     // Register the platform-specific implementation of the idiomatic
     // InAppPurchase API.
-    InAppPurchasePlatform.setInstance(InAppPurchaseIosPlatform());
+    InAppPurchasePlatform.instance = InAppPurchaseIosPlatform();
 
     _skPaymentQueueWrapper = SKPaymentQueueWrapper();
     _observer = _TransactionObserver(StreamController.broadcast());

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/errors/errors.dart

@@ -2,4 +2,4 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-export 'errors.dart';
+export 'in_app_purchase_exception.dart';