SetTmutilExclusion: ignore failures to add or remove exclusions caused by signal 9

[cole-h]

This is probably maybe caused by a macOS policy disabling TimeMachine.

Description

Closes #1435.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

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