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`.
actually nah, looks fine to me, just maybe do:

```js
err.export = 'number' == typeof err.status && http.STATUS_CODES[err.status] && err.status < 500
```