Fix error when users.table does not exist yet during initialize

[KevinMind]

Fixes: mozilla/addons#15196

Follow up: #22860

Description

Add error handling to check_users in the initialize script

Context

When running initialize we check if there are any users in the user table.. however if you run this command before the database has been migrated, it will raise a mysql error.

We should handle that such that if the query errors, we assume no users and return false

Testing

make down
make docker_mysqld_volume_remove
make up

Should not error

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.