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/build/jest.js

@@ -0,0 +1,44 @@
+'use strict';
+
+function cli() {
+  const data = _interopRequireWildcard(require('jest-cli'));
+
+  cli = function cli() {
+    return data;
+  };
+
+  return data;
+}
+
+function _interopRequireWildcard(obj) {
+  if (obj && obj.__esModule) {
+    return obj;
+  } else {
+    var newObj = {};
+    if (obj != null) {
+      for (var key in obj) {
+        if (Object.prototype.hasOwnProperty.call(obj, key)) {
+          var desc =
+            Object.defineProperty && Object.getOwnPropertyDescriptor
+              ? Object.getOwnPropertyDescriptor(obj, key)
+              : {};
+          if (desc.get || desc.set) {
+            Object.defineProperty(newObj, key, desc);
+          } else {
+            newObj[key] = obj[key];
+          }
+        }
+      }
+    }
+    newObj.default = obj;
+    return newObj;
+  }
+}
+
+/**
+ * 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.
+ */
+module.exports = cli();