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/shebang-regex/readme.md

@@ -0,0 +1,29 @@
+# shebang-regex [![Build Status](https://travis-ci.org/sindresorhus/shebang-regex.svg?branch=master)](https://travis-ci.org/sindresorhus/shebang-regex)
+
+> Regular expression for matching a [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix))
+
+
+## Install
+
+```
+$ npm install --save shebang-regex
+```
+
+
+## Usage
+
+```js
+var shebangRegex = require('shebang-regex');
+var str = '#!/usr/bin/env node\nconsole.log("unicorns");';
+
+shebangRegex.test(str);
+//=> true
+
+shebangRegex.exec(str)[0];
+//=> '#!/usr/bin/env node'
+```
+
+
+## License
+
+MIT © [Sindre Sorhus](http://sindresorhus.com)