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GH-39579: [Python] fix raising ValueError on _ensure_partitioning (#39593)



### Rationale for this change
The `_ensure_partitioning` method in dataset.py is missing a "raise" which currently ignores bad scheme silently.

### What changes are included in this PR?

Fixed the typo.

### Are these changes tested?
Tried with new code that the exception is properly raised.

### Are there any user-facing changes?
No.

* Closes: #39579

Lead-authored-by: idailylife <iorange@126.com>
Co-authored-by: 0x0000ffff <idailylife@users.noreply.github.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>