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/number-is-nan/readme.md

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+# number-is-nan [![Build Status](https://travis-ci.org/sindresorhus/number-is-nan.svg?branch=master)](https://travis-ci.org/sindresorhus/number-is-nan)
+
+> ES2015 [`Number.isNaN()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN) [ponyfill](https://ponyfill.com)
+
+
+## Install
+
+```
+$ npm install --save number-is-nan
+```
+
+
+## Usage
+
+```js
+var numberIsNan = require('number-is-nan');
+
+numberIsNan(NaN);
+//=> true
+
+numberIsNan('unicorn');
+//=> false
+```
+
+
+## License
+
+MIT © [Sindre Sorhus](http://sindresorhus.com)