feat: return cluster error messages on wait handler

[h3adex]

Description

This change aligns with the corresponding Terraform PR stackitcloud/terraform-provider-stackit#1010. It updates the CreateOrUpdateClusterWaitHandler in the ske/wait package to exit early when the cluster reports structured errors and is in a failure state. This prevents cluster creation or update operations from remaining stuck in an error state for up to 45 minutes until the Terraform provider times out.

The related Terraform logic is in the PR linked above. Before merging this PR, someone should also review the Terraform changes. Test SKE configurations are attached for verification.

Result warning trying to create a ske cluster using terraform with a non existent flavor for machines

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs
  • Changelog in the root directory was adjusted (see here)
  • Changelog(s) of the service(s) were adjusted (see e.g. here)
• VERSION file(s) of the service(s) were adjusted
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

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