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/path-key/readme.md

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+# path-key [![Build Status](https://travis-ci.org/sindresorhus/path-key.svg?branch=master)](https://travis-ci.org/sindresorhus/path-key)
+
+> Get the [PATH](https://en.wikipedia.org/wiki/PATH_(variable)) environment variable key cross-platform
+
+It's usually `PATH`, but on Windows it can be any casing like `Path`...
+
+
+## Install
+
+```
+$ npm install --save path-key
+```
+
+
+## Usage
+
+```js
+const pathKey = require('path-key');
+
+const key = pathKey();
+//=> 'PATH'
+
+const PATH = process.env[key];
+//=> '/usr/local/bin:/usr/bin:/bin'
+```
+
+
+## API
+
+### pathKey([options])
+
+#### options
+
+##### env
+
+Type: `Object`<br>
+Default: [`process.env`](https://nodejs.org/api/process.html#process_process_env)
+
+Use a custom environment variables object.
+
+#### platform
+
+Type: `string`<br>
+Default: [`process.platform`](https://nodejs.org/api/process.html#process_process_platform)
+
+Get the PATH key for a specific platform.
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)