add error event

modules/imagery/provider/BaiduImageryProvider.js

     this._credit = undefined
     this._token = undefined
     this._style = options.style || 'normal'
+    this._errorEvent = new Cesium.Event()
   }
 
   get url() {
     return true
   }
 
+  get errorEvent() {
+    return this._errorEvent
+  }
+
   getTileCredits(x, y, level) {}
 
   /**