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/_arrayShuffle.js

@@ -0,0 +1,15 @@
+var copyArray = require('./_copyArray'),
+    shuffleSelf = require('./_shuffleSelf');
+
+/**
+ * A specialized version of `_.shuffle` for arrays.
+ *
+ * @private
+ * @param {Array} array The array to shuffle.
+ * @returns {Array} Returns the new shuffled array.
+ */
+function arrayShuffle(array) {
+  return shuffleSelf(copyArray(array));
+}
+
+module.exports = arrayShuffle;