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/import-local/readme.md

@@ -0,0 +1,30 @@
+# import-local [![Build Status](https://travis-ci.org/sindresorhus/import-local.svg?branch=master)](https://travis-ci.org/sindresorhus/import-local)
+
+> Let a globally installed package use a locally installed version of itself if available
+
+Useful for CLI tools that want to defer to the user's locally installed version when available, but still work if it's not installed locally. For example, [AVA](http://ava.li) and [XO](https://github.com/xojs/xo) uses this method.
+
+
+## Install
+
+```
+$ npm install import-local
+```
+
+
+## Usage
+
+```js
+const importLocal = require('import-local');
+
+if (importLocal(__filename)) {
+	console.log('Using local version of this package');
+} else {
+	// Code for both global and local version here…
+}
+```
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)