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-diff/build/diffStrings.d.ts

@@ -0,0 +1,14 @@
+/**
+ * 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 { DiffOptions } from './types';
+declare type Original = {
+    a: string;
+    b: string;
+};
+declare const _default: (a: string, b: string, options?: DiffOptions | undefined, original?: Original | undefined) => string;
+export default _default;
+//# sourceMappingURL=diffStrings.d.ts.map