Added missing license header

Author: mvanbeusekom

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

@@ -1 +1,5 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 export 'purchase_exception.dart';