Fix error display for Split by Chapter (#3621)

## Summary
- throw `IllegalArgumentException` when bookmark level is invalid or
when a PDF has no outline
- rely on global error handling so frontend shows the message

## Testing
- `./gradlew build`


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https://chatgpt.com/codex/tasks/task_b_683dc51dd31083288be3f9892889fa59

Anthony Stirling committed 1y ago

5d9d8a5625248286ca10c3d629cc1cb13b56d3cb

Parent: a8c6a83

Committed by GitHub <noreply@github.com> on 6/3/2025, 4:44:35 PM