So it seems that in order to make this work, I need to rewrite the entire `defer` block manually (making the request and rendering the result myself), since there isn't a way to listen for a failure in a `defer` (let alone a specific error code). I think `defer` should be able to support this, though — it might be worth enhancing it at some point to dispatch events on success and failure with the request results attached to the event, so we can easily update other aspects of the page when a `defer` block finishes loading.