Failed custom hook output not shown on Merge Request

Reproduced in version 11.4.0-ee

Similar to #1652 (closed) and #1852 (closed) creating a simple custom pre-receive hook with the contents

#!/bin/bash
echo Testing custom hook!
exit 1

and then trying to merge a Merge Request, I expect the output Testing custom hook! on the Merge request page, as documented here: https://docs.gitlab.com/11.3/ee/administration/custom_hooks.html#custom-error-messages

Instead, only "Something went wrong during merge pre-receive hook. Refreshing in 10 seconds to show the updated status..." is shown, similar to #7212 (closed)