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/shebang-command/readme.md

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+# shebang-command [![Build Status](https://travis-ci.org/kevva/shebang-command.svg?branch=master)](https://travis-ci.org/kevva/shebang-command)
+
+> Get the command from a shebang
+
+
+## Install
+
+```
+$ npm install --save shebang-command
+```
+
+
+## Usage
+
+```js
+const shebangCommand = require('shebang-command');
+
+shebangCommand('#!/usr/bin/env node');
+//=> 'node'
+
+shebangCommand('#!/bin/bash');
+//=> 'bash'
+```
+
+
+## API
+
+### shebangCommand(string)
+
+#### string
+
+Type: `string`
+
+String containing a shebang.
+
+
+## License
+
+MIT © [Kevin Martensson](http://github.com/kevva)