[Data] Clarify schema validation error

[bveeramani]

Why are these changes needed?

ray.data.range(1).groupby("does_not_exist").count().materialize()

Before

ValueError: The column 'does_not_exist' does not exist in the schema 'Column  Type
------  ----
id      int64'.

After

ValueError: You specified the column 'does_not_exist', but there's no such column in the dataset. The dataset has columns: {'id'}

Related issue number

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[alexeykudinkin]

I think it's better to print full schema (though fixing the formatting)