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/escape-string-regexp/readme.md

@@ -0,0 +1,27 @@
+# escape-string-regexp [![Build Status](https://travis-ci.org/sindresorhus/escape-string-regexp.svg?branch=master)](https://travis-ci.org/sindresorhus/escape-string-regexp)
+
+> Escape RegExp special characters
+
+
+## Install
+
+```
+$ npm install --save escape-string-regexp
+```
+
+
+## Usage
+
+```js
+const escapeStringRegexp = require('escape-string-regexp');
+
+const escapedString = escapeStringRegexp('how much $ for a unicorn?');
+//=> 'how much \$ for a unicorn\?'
+
+new RegExp(escapedString);
+```
+
+
+## License
+
+MIT © [Sindre Sorhus](http://sindresorhus.com)