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/is-generator-fn/readme.md

@@ -0,0 +1,33 @@
+# is-generator-fn [![Build Status](https://travis-ci.org/sindresorhus/is-generator-fn.svg?branch=master)](https://travis-ci.org/sindresorhus/is-generator-fn)
+
+> Check if something is a [generator function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*)
+
+
+## Install
+
+```
+$ npm install is-generator-fn
+```
+
+
+## Usage
+
+```js
+const isGeneratorFn = require('is-generator-fn');
+
+isGeneratorFn(function * () {});
+//=> true
+
+isGeneratorFn(function () {});
+//=> false
+```
+
+
+## Related
+
+- [is](https://github.com/sindresorhus/is) - Type check values
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)