Fix off-by-one assertion and catch UnknownIndexException

[driftx]

What is the issue

When the guardrail index limit equals the number of excluded indexes (as in both are 1), the assertion 1 < 1 fails even though having exactly the maximum allowed should be valid.

What does this PR fix and why was it fixed

Fixes the assertion and catches UnknownIndexException

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits
• All new files should contain the DataStax copyright header instead of the Apache License one

[driftx]

Just noticed main has the same problem /cc @adelapena

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
71.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

✔️ Build ds-cassandra-pr-gate/PR-2154 approved by Butler

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Approved by Butler
See build details here