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-fullwidth-code-point/readme.md

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+# is-fullwidth-code-point [![Build Status](https://travis-ci.org/sindresorhus/is-fullwidth-code-point.svg?branch=master)](https://travis-ci.org/sindresorhus/is-fullwidth-code-point)
+
+> Check if the character represented by a given [Unicode code point](https://en.wikipedia.org/wiki/Code_point) is [fullwidth](https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms)
+
+
+## Install
+
+```
+$ npm install --save is-fullwidth-code-point
+```
+
+
+## Usage
+
+```js
+const isFullwidthCodePoint = require('is-fullwidth-code-point');
+
+isFullwidthCodePoint('谢'.codePointAt());
+//=> true
+
+isFullwidthCodePoint('a'.codePointAt());
+//=> false
+```
+
+
+## API
+
+### isFullwidthCodePoint(input)
+
+#### input
+
+Type: `number`
+
+[Code point](https://en.wikipedia.org/wiki/Code_point) of a character.
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)