Steps to reproduce:

1. Create any Landing Page.
2. Add 3 blocks to it.
3. Publish.
4. Visit this Landing Page from step 1 in a frontend - use Symfony's debug mode.
5. Go to the last 10 requests section and save blocks' ids from the _fragment requests:
   
6. Break a single block, for example - go to vendor/ibexa/fieldtype-page/src/bundle/Controller/BlockController.php file and in line with $event do something like:

   $event = new BlockResponseEvent($blockId == 40 ? null : $blockContext, $blockValue, $request, new Response());
   

   where 40 is an ID of a block that we want to kill.

       7. Clear a cache and re-visit the Landing Page in a frontend.

Result:
One can see that a 500 exception is thrown, and no block is rendered.

Expected result:
The remaining blocks should be rendered properly and the broken block should display an error message. 

Note:
Seems like blocks are failing gracefully in v3.3 so this looks like a regression.