fix an error log (#399)

Author: wangxiao

src/utils.js

@@ -230,7 +230,7 @@ const init = (AV) => {
    */
   AV._getAVPath = function(path) {
     if (!AV.applicationId) {
-      throw "You need to call AV.initialize before using AV.";
+      throw new Error('You need to call AV.init() before using AV.');
     }
     if (!path) {
       path = "";