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/core/build/pluralize.js

@@ -0,0 +1,11 @@
+'use strict';
+
+Object.defineProperty(exports, '__esModule', {
+  value: true
+});
+exports.default = pluralize;
+
+// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+function pluralize(word, count, ending) {
+  return `${count} ${word}${count === 1 ? '' : ending}`;
+}