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-runtime/bin/jest-runtime.js

@@ -0,0 +1,13 @@
+#!/usr/bin/env node
+/**
+ * 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.
+ */
+
+if (process.env.NODE_ENV == null) {
+  process.env.NODE_ENV = 'test';
+}
+
+require('../build/cli').run();