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/es-abstract/README.md

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+# es-abstract <sup>[![Version Badge][npm-version-svg]][package-url]</sup>
+
+[![Build Status][travis-svg]][travis-url]
+[![dependency status][deps-svg]][deps-url]
+[![dev dependency status][dev-deps-svg]][dev-deps-url]
+[![License][license-image]][license-url]
+[![Downloads][downloads-image]][downloads-url]
+
+[![npm badge][npm-badge-png]][package-url]
+
+[![browser support][testling-svg]][testling-url]
+
+ECMAScript spec abstract operations.
+When different versions of the spec conflict, the default export will be the latest version of the abstract operation.
+All abstract operations will also be available under an `es5`/`es2015`/`es2016`/`es2017`/`es2018` entry point, and exported property, if you require a specific version.
+
+## Example
+
+```js
+var ES = require('es-abstract');
+var assert = require('assert');
+
+assert(ES.isCallable(function () {}));
+assert(!ES.isCallable(/a/g));
+```
+
+## Tests
+Simply clone the repo, `npm install`, and run `npm test`
+
+[package-url]: https://npmjs.org/package/es-abstract
+[npm-version-svg]: http://versionbadg.es/ljharb/es-abstract.svg
+[travis-svg]: https://travis-ci.org/ljharb/es-abstract.svg
+[travis-url]: https://travis-ci.org/ljharb/es-abstract
+[deps-svg]: https://david-dm.org/ljharb/es-abstract.svg
+[deps-url]: https://david-dm.org/ljharb/es-abstract
+[dev-deps-svg]: https://david-dm.org/ljharb/es-abstract/dev-status.svg
+[dev-deps-url]: https://david-dm.org/ljharb/es-abstract#info=devDependencies
+[testling-svg]: https://ci.testling.com/ljharb/es-abstract.png
+[testling-url]: https://ci.testling.com/ljharb/es-abstract
+[npm-badge-png]: https://nodei.co/npm/es-abstract.png?downloads=true&stars=true
+[license-image]: https://img.shields.io/npm/l/es-abstract.svg
+[license-url]: LICENSE
+[downloads-image]: https://img.shields.io/npm/dm/es-abstract.svg
+[downloads-url]: https://npm-stat.com/charts.html?package=es-abstract