fix: print API error message instead of operation nil

[mlec1]

Description

Exactly like in #806, it returns operation is nil when update a SKSNodePool when it is called with parameters that the API rejects.

exo compute sks nodepool update itops-test itops-test-workers --label managed_uptime=standard --label dont_manage_until=2026-05-06 --label test=test/test

At the opposite of #806, I didn't add a e2e test because I encountered the issue while trying to add a / in a label of a SKS Nodepool but at the end I would like that it works (I made a support ticket for this), so if I make a E2E test for it now, it might not pass anymore once / are accepted on the API Side.

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block, and add the Pull Request #number for each bit you add to the CHANGELOG.md)
• Testing

Testing

Now returns the BAD Request response from the API.