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/collection-visit/index.js

@@ -0,0 +1,30 @@
+/*!
+ * collection-visit <https://github.com/jonschlinkert/collection-visit>
+ *
+ * Copyright (c) 2015, 2017, Jon Schlinkert.
+ * Released under the MIT License.
+ */
+
+'use strict';
+
+var visit = require('object-visit');
+var mapVisit = require('map-visit');
+
+module.exports = function(collection, method, val) {
+  var result;
+
+  if (typeof val === 'string' && (method in collection)) {
+    var args = [].slice.call(arguments, 2);
+    result = collection[method].apply(collection, args);
+  } else if (Array.isArray(val)) {
+    result = mapVisit.apply(null, arguments);
+  } else {
+    result = visit.apply(null, arguments);
+  }
+
+  if (typeof result !== 'undefined') {
+    return result;
+  }
+
+  return collection;
+};