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/_baseUpdate.js

@@ -0,0 +1,18 @@
+var baseGet = require('./_baseGet'),
+    baseSet = require('./_baseSet');
+
+/**
+ * The base implementation of `_.update`.
+ *
+ * @private
+ * @param {Object} object The object to modify.
+ * @param {Array|string} path The path of the property to update.
+ * @param {Function} updater The function to produce the updated value.
+ * @param {Function} [customizer] The function to customize path creation.
+ * @returns {Object} Returns `object`.
+ */
+function baseUpdate(object, path, updater, customizer) {
+  return baseSet(object, path, updater(baseGet(object, path)), customizer);
+}
+
+module.exports = baseUpdate;