Add error description when lock room fails (#911)

* Add error description

* Change error message

* Change error message

JitsiConference.js

@@ -1086,7 +1086,7 @@ JitsiConference.prototype.isModerator = function() {
  */
 JitsiConference.prototype.lock = function(password) {
     if (!this.isModerator()) {
-        return Promise.reject();
+        return Promise.reject(new Error('You are not moderator.'));
     }
 
     return new Promise((resolve, reject) => {