Response object is not sent to callback

It looks like the response object is not sent back to callback when an exception is thrown:

```
this.on('end', function () {
var err = null;
var res = null;

try {
res = new Response(self);
} catch (err_) {
// ...
return self.callback(err); // <-- HERE
}
// ...
});
```

Start by locating the `end` handler containing `new Response(self)` and the `self.callback(err)` line shown in the issue. Trace the callback contract around this exception path; done means the callback receives the response object when the exception is thrown, without changing other error handling.

Written by the indexing model from the issue text.