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/qs/lib/formats.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var replace = String.prototype.replace;
+var percentTwenties = /%20/g;
+
+module.exports = {
+    'default': 'RFC3986',
+    formatters: {
+        RFC1738: function (value) {
+            return replace.call(value, percentTwenties, '+');
+        },
+        RFC3986: function (value) {
+            return value;
+        }
+    },
+    RFC1738: 'RFC1738',
+    RFC3986: 'RFC3986'
+};