Improve noble migration check script output

[legoktm]

Status

Ready for review

Description of Changes

• Explicitly flag which messages are error conditions; otherwise it would be hard to tell whether "group is empty" or "group is not empty" is the problematic state.
• Do the same for success messages by adding "OK" to them.
• Add a newline before our closing help message.
• Link to docs that explain how to resolve these issues (still pending review at Document how to resolve noble pre-migration issues securedrop-docs#612).

Testing

How should the reviewer test this PR?

• visual review; sample output can be found in Document how to resolve noble pre-migration issues securedrop-docs#612

Deployment

Any special considerations for deployment? n/a

Checklist

• Linting (make lint) and tests (make test) pass in the development container

[legoktm]

While working on the documentation PR (freedomofpress/securedrop-docs#612), I realized the output wasn't that great, so these are some minor changes to improve it.

[cfm]

This looks good to me. Non-blocking nit: I would favor going all the way to (e.g.)—

• ssh OK: group is empty
• ssh ERROR: group is not empty [...]

—so that success as well as failure is explicit rather than implicit. But I defer to you.

[legoktm]

That's a good idea, give me a minute :)

[cfm]

Lovely. Thanks, @legoktm.