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/invert-kv/readme.md

@@ -0,0 +1,25 @@
+# invert-kv [![Build Status](https://travis-ci.org/sindresorhus/invert-kv.svg?branch=master)](https://travis-ci.org/sindresorhus/invert-kv)
+
+> Invert the key/value of an object. Example: `{foo: 'bar'}` → `{bar: 'foo'}`
+
+
+## Install
+
+```
+$ npm install invert-kv
+```
+
+
+## Usage
+
+```js
+const invertKv = require('invert-kv');
+
+invertKv({foo: 'bar', unicorn: 'rainbow'});
+//=> {bar: 'foo', rainbow: 'unicorn'}
+```
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)