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/_baseSortBy.js

@@ -0,0 +1,21 @@
+/**
+ * The base implementation of `_.sortBy` which uses `comparer` to define the
+ * sort order of `array` and replaces criteria objects with their corresponding
+ * values.
+ *
+ * @private
+ * @param {Array} array The array to sort.
+ * @param {Function} comparer The function to define sort order.
+ * @returns {Array} Returns `array`.
+ */
+function baseSortBy(array, comparer) {
+  var length = array.length;
+
+  array.sort(comparer);
+  while (length--) {
+    array[length] = array[length].value;
+  }
+  return array;
+}
+
+module.exports = baseSortBy;