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/invoke.js

@@ -0,0 +1,24 @@
+var baseInvoke = require('./_baseInvoke'),
+    baseRest = require('./_baseRest');
+
+/**
+ * Invokes the method at `path` of `object`.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Object
+ * @param {Object} object The object to query.
+ * @param {Array|string} path The path of the method to invoke.
+ * @param {...*} [args] The arguments to invoke the method with.
+ * @returns {*} Returns the result of the invoked method.
+ * @example
+ *
+ * var object = { 'a': [{ 'b': { 'c': [1, 2, 3, 4] } }] };
+ *
+ * _.invoke(object, 'a[0].b.c.slice', 1, 3);
+ * // => [2, 3]
+ */
+var invoke = baseRest(baseInvoke);
+
+module.exports = invoke;