* filer: fail writes when the existing-entry lookup fails
CreateEntry only returned a FindEntry error for exclusive creates; a
regular write treated a transient store error as "entry not found" and,
on upsert stores, replaced the entry without loading its old chunks —
leaving them out of the cleanup process. Return any lookup error except
ErrNotFound for both paths.
Generated with [Devin](https://devin.ai)
Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
* filer: test that a lookup failure fails a regular write
Generated with [Devin](https://devin.ai)
Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
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Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>