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/lodash/shuffle.js

@@ -0,0 +1,25 @@
+var arrayShuffle = require('./_arrayShuffle'),
+    baseShuffle = require('./_baseShuffle'),
+    isArray = require('./isArray');
+
+/**
+ * Creates an array of shuffled values, using a version of the
+ * [Fisher-Yates shuffle](https://en.wikipedia.org/wiki/Fisher-Yates_shuffle).
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Collection
+ * @param {Array|Object} collection The collection to shuffle.
+ * @returns {Array} Returns the new shuffled array.
+ * @example
+ *
+ * _.shuffle([1, 2, 3, 4]);
+ * // => [4, 1, 3, 2]
+ */
+function shuffle(collection) {
+  var func = isArray(collection) ? arrayShuffle : baseShuffle;
+  return func(collection);
+}
+
+module.exports = shuffle;