Fail when explicitly deleted data file is missing

[rambleraptor]

Rationale for this change

Fixes #3814
This looks like the problem I pointed out in #3811 but didn't explain properly enough. We should look for all explicitly deleted files and make sure they exist. This matches how Java handles the situation.

Are these changes tested?

Included unit tests based off the issue's verification script

Are there any user-facing changes?

[kevinjqliu]

LGTM! Thanks for the fix.

I pushed up a commit based on review with codex. Its mostly simple refactor of your existing code.
The only other significant change is in _write_delete_manifest: moving self._deleted_entries() out into the parent will run the validation code before we start writing manifests

[qzyu999]

Hi @rambleraptor, nice fix! While reviewing this I noticed a related pre-existing gap: _DeleteFiles (the update_snapshot().delete() path) exposes delete_data_file() from its parent class, but calling it silently does nothing because _compute_deletes resets self._deleted_data_files = set() before scanning by predicate.

This means _OverwriteFiles correctly validates explicit deletes (thanks to this PR), but _DeleteFiles silently drops them.

Filed as #3857 with a reproduction script.