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/has/README.md

@@ -0,0 +1,18 @@
+# has
+
+> Object.prototype.hasOwnProperty.call shortcut
+
+## Installation
+
+```sh
+npm install --save has
+```
+
+## Usage
+
+```js
+var has = require('has');
+
+has({}, 'hasOwnProperty'); // false
+has(Object.prototype, 'hasOwnProperty'); // true
+```