Image组件设置使用网络图片，如果网络图片获取不成功，应该如何设置兜底图片

@Entry 
@Component 
struct Page3 { 
  @State message: string = 'Hello World' 
  @State alt: Resource = $r('app.media.app_icon') 
 
  build() { 
    Row() { 
      Column() { 
        Image('https://l.sinaimg.cn/wx1/original/aae7c86cgy1hgppngx0hwg21400u07wp.gif/original.webp') 
          .onError(() => { 
            console.log('demoTest') 
            this.alt = $r('app.media.img') 
          }) 
          .alt(this.alt) 
      } 
      .width('100%') 
    } 
    .height('100%') 
  } 
}