Don't check for error if event is not stopped

It's a very minor improvement, related to https://github.com/zendframework/zf2/commit/26bc1ea6da647d92a6a57bc1d71a0d5cb3177dbe.

Since `$shortCircuit` is:
```php
$shortCircuit = function ($r) use ($event) {
    if ($r instanceof ResponseInterface) {
        return true;
    }
    if ($event->getError()) {
        return true;
    }
    return false;
};
```
We can assume that if the event is not stopped, then there's no error in `MvcEvent`.