How can I convert the code that used drupal_get_http_header() to check if a 403 or 404 error is returned?
You can subscribe to the Symfony KernelEvents::RESPONSE event:

The RESPONSE event occurs once a response was created for replying to a request
This event allows you to modify or replace the response that will be replied. The event listener method receives a Symfony\Component\HttpKernel\Event\FilterResponseEvent instance.

To do so, create a new module (&quot;response_test&quot; in this example), with the usual .info.yml and .module file.
Then register your event subscriber in the module's services file:
response_test.services.yml
services:
    response_test.response_subscriber:
      class: Drupal\response_test\ResponseSubscriber
      tags:
        - { name: event_subscriber }

Now all you need is the class to handle the event:
lib/Drupal/response_test/ResponseSubscriber.php
&lt;?php 

namespace Drupal\response_test;

use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class ResponseSubscriber implements EventSubscriberInterface {
  
  public function onRespond(FilterResponseEvent $event) {
    if ($event-&gt;getRequestType() !== HttpKernelInterface::MASTER_REQUEST) {
      return;
    }
    
    $response = $event-&gt;getResponse();
    if ($response-&gt;getStatusCode() == 404) {
      // Prepare a new Symfony\Component\HttpFoundation\Response and use
      $event-&gt;setResponse($new_response);

      // or call one of the setter methods on $response, the changes will persist
    }
  }
  
  public static function getSubscribedEvents() {
    $events[KernelEvents::RESPONSE][] = array('onRespond');
    return $events;
  }
  
}

I'm a little unsure about changing the HTML content in this context; you have access to $response-&gt;getContent() and $response-&gt;setContent() so you can manipulate the raw string directly, but as far as I can tell everything's already rendered at this point.