koajs/koa — pull request #330 by dead-horse (2014-08-06 13:40:58+00:00)
Title: Onerror status

```
var err = new Error('msg');
err.status = -1;

throw err;
```

`ctx.onerror` call `response.status = err.status`, lead to `uncaughtException`.

```
var err = new Error('msg');
err.message = -1;

this.throw(err);
```

will set `err.expose = true`.