Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/jest-snapshot/build/mock_serializer.d.ts

@@ -0,0 +1,12 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+import { NewPlugin } from 'pretty-format';
+export declare const serialize: NewPlugin['serialize'];
+export declare const test: NewPlugin['test'];
+declare const plugin: NewPlugin;
+export default plugin;
+//# sourceMappingURL=mock_serializer.d.ts.map