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/repeat-element/index.js

@@ -0,0 +1,18 @@
+/*!
+ * repeat-element <https://github.com/jonschlinkert/repeat-element>
+ *
+ * Copyright (c) 2015-present, Jon Schlinkert.
+ * Licensed under the MIT license.
+ */
+
+'use strict';
+
+module.exports = function repeat(ele, num) {
+  var arr = new Array(num);
+
+  for (var i = 0; i < num; i++) {
+    arr[i] = ele;
+  }
+
+  return arr;
+};