Fix GPU error messages for fuzzy deduplication

[sarahyurick]

• Raise a clear and readable error if the Dask client is not GPU-based
• Raise a clear and readable error if the dataset’s backend is not cuDF

[sarahyurick]

cc @Maghoumi please let me know if these error messages look more how you would expect them.

[Maghoumi]

Since this pattern is repeated, it might be better to move to a separate helper (e.g. checks.py) so it can be re-used in different places.

Something like: ensure_cudf(dataset, module="buckets_to_edges") which would print something like:

A Dask DataFrame with cuDF backend is required to run 'buckets_to_edges'. Please convert using....

[sarahyurick]

Thanks, I see what you mean but for now I'm not sure whether it's worth adding a new script for this. I will ask others to review as well, thank you.