Fix dataset level aggregators
copy the error if the previous step is erroneous, instead of errors that cannot be understood by the user, like https://github.com/huggingface/datasets-server/issues/1055. That way, an error (like 'empty dataset' for example) is propagated to the descendants.

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Before, on an empty dataset repository, the dataset-split-names, for example, returned:

<img width="671" alt="Capture d’écran 2023-04-18 à 16 47 01" src="https://user-images.githubusercontent.com/1676121/232814116-bbf9ae08-c4fa-4b3c-ae3d-1330a0d4ba5b.png">


Now:

<img width="1592" alt="Capture d’écran 2023-04-18 à 16 45 05" src="https://user-images.githubusercontent.com/1676121/232813719-67b8e8c4-89be-4d50-9607-02d1aab0843c.png">

Fix dataset level aggregators: copy the error if the previous step is erroneous, instead of errors that cannot be understood by the user, like https://github.com/huggingface/datasets-server/issues/1055. That way, an error (like 'empty dataset' for example) is propagated to the descendants.

---

Before, on an empty dataset repository, the dataset-split-names, for example, returned:

<img width="671" alt="Capture d’écran 2023-04-18 à 16 47 01" src="https://user-images.githubusercontent.com/1676121/232814116-bbf9ae08-c4fa-4b3c-ae3d-1330a0d4ba5b.png">


Now:

<img width="1592" alt="Capture d’écran 2023-04-18 à 16 45 05" src="https://user-images.githubusercontent.com/1676121/232813719-67b8e8c4-89be-4d50-9607-02d1aab0843c.png">
