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/decamelize/readme.md

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+# decamelize [![Build Status](https://travis-ci.org/sindresorhus/decamelize.svg?branch=master)](https://travis-ci.org/sindresorhus/decamelize)
+
+> Convert a camelized string into a lowercased one with a custom separator<br>
+> Example: `unicornRainbow` → `unicorn_rainbow`
+
+
+## Install
+
+```
+$ npm install --save decamelize
+```
+
+
+## Usage
+
+```js
+const decamelize = require('decamelize');
+
+decamelize('unicornRainbow');
+//=> 'unicorn_rainbow'
+
+decamelize('unicornRainbow', '-');
+//=> 'unicorn-rainbow'
+```
+
+
+## API
+
+### decamelize(input, [separator])
+
+#### input
+
+Type: `string`
+
+#### separator
+
+Type: `string`<br>
+Default: `_`
+
+
+## Related
+
+See [`camelcase`](https://github.com/sindresorhus/camelcase) for the inverse.
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)