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/_baseAt.js

@@ -0,0 +1,23 @@
+var get = require('./get');
+
+/**
+ * The base implementation of `_.at` without support for individual paths.
+ *
+ * @private
+ * @param {Object} object The object to iterate over.
+ * @param {string[]} paths The property paths to pick.
+ * @returns {Array} Returns the picked elements.
+ */
+function baseAt(object, paths) {
+  var index = -1,
+      length = paths.length,
+      result = Array(length),
+      skip = object == null;
+
+  while (++index < length) {
+    result[index] = skip ? undefined : get(object, paths[index]);
+  }
+  return result;
+}
+
+module.exports = baseAt;