what's the meaning about listening the finished event of response? 
In Line 167#application.js, there have the code:
```
line162:    handleRequest(ctx, fnMiddleware) {
line163:      const res = ctx.res;
line164:        res.statusCode = 404;
line165:        const onerror = err => ctx.onerror(err);
line166:        const handleResponse = () => respond(ctx);
line167:        onFinished(res, onerror);
line168:        return fnMiddleware(ctx).then(handleResponse).catch(onerror);
line169:     }
```
`onFinished(res, onerror)` can add a listener on the `error`/`finished` event of res. 
And then, the listener below in Line 107#context.js:
```
  onerror(err) {
    if (null == err) return;
    //... other codes
  }
```
But in `finished` event,  the `null == err` always true. So the code is always returned.It's make me confused.
So would you have a time can described the meaning ? Thanks very much!

