[infra] feat: notify Discord role when URLs checks fail

[GresilleSiffle]

related issues #1822

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Description

This PR makes the external URLs monitoring script able to highlight a specific Discord role when it fails.

The goal is to explicitly and quickly notify the core team when the staging and production server are unreachable.

Checklist

• I added the related issue(s) id in the related issues section (if any)
  • if not, delete the related issues section
• I described my changes and my decisions in the PR description
• I read the development guidelines of the CONTRIBUTING.md
• The tests pass and have been updated if relevant
• The code quality check pass

[GresilleSiffle]

In the current state, if /root/discord_role_id_urls_checks doesn't exist the command will fail and will stop the deployment.

If we want to keep this feature optional, we could use:

# won't fail, the error will be displayed in the stdout
sudo cat /root/discord_role_id_urls_checks || true

# won't fail, the error will be silent
sudo cat /root/discord_role_id_urls_checks 2> /dev/null || true

In theory I think that DISCORD_ROLE_ID_URLS_CHECKS should be optional, to let people deploy a Tournesol instance without being forced to use Discord alerts. Thus using 2> /dev/null could be appropriate if we don't want to display an error when an optional setting is missing.

What do you think @amatissart ?

[amatissart]

2> /dev/null is fine.

Alternatively, I think it would be acceptable to define the variable discord_role_id_urls_checks publicly in "inventory.yml" in the tournesol-prod section.

[GresilleSiffle]

Sure, addressed in:

• ed9bac3
• 55cfb17