Improve error message of disable_sign_ups command

Context

In https://gitlab.com/gitlab-org/gitlab/-/jobs/3615102128, we sometimes see the following error:

command terminated with exit code 137

We do not know what the error is about.

Goal

Have some more information about the source of the failure. I have the feeling we are swallowing the error before displaying the error code. Ideally, we would show the error message in the job logs.