v2 app.on('error') confusing
This sample application only returns 500.
```javascript
const Koa = require('koa');

const app = new Koa();
app.on('error', (err, ctx) => {
  console.log(err);
  console.log(ctx.status);
});
app.use(() => {
  throw new Error('Terrible Error');
});
app.listen(3000);
```

Output:
```
Error: Terrible Error
    ... error stack etc..
404
```

Why is ctx.status 404 in the app.on 'error'?