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-stream/readme.md

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+# is-stream [![Build Status](https://travis-ci.org/sindresorhus/is-stream.svg?branch=master)](https://travis-ci.org/sindresorhus/is-stream)
+
+> Check if something is a [Node.js stream](https://nodejs.org/api/stream.html)
+
+
+## Install
+
+```
+$ npm install --save is-stream
+```
+
+
+## Usage
+
+```js
+const fs = require('fs');
+const isStream = require('is-stream');
+
+isStream(fs.createReadStream('unicorn.png'));
+//=> true
+
+isStream({});
+//=> false
+```
+
+
+## API
+
+### isStream(stream)
+
+#### isStream.writable(stream)
+
+#### isStream.readable(stream)
+
+#### isStream.duplex(stream)
+
+#### isStream.transform(stream)
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)