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/domexception/README.md

@@ -0,0 +1,19 @@
+# DOMException
+
+This package implements the [`DOMException`](https://heycam.github.io/webidl/#idl-DOMException) class, from web browsers. It exists in service of [jsdom](https://github.com/tmpvar/jsdom) and related packages.
+
+Example usage:
+
+```js
+const DOMException = require("domexception");
+
+const e1 = new DOMException("Something went wrong", "BadThingsError");
+console.assert(e1.name === "BadThingsError");
+console.assert(e1.code === 0);
+
+const e2 = new DOMException("Another exciting error message", "NoModificationAllowedError");
+console.assert(e2.name === "NoModificationAllowedError");
+console.assert(e2.code === 7);
+
+console.assert(DOMException.INUSE_ATTRIBUTE_ERR === 10);
+```