[Datasets] Improve iter(dataset) error message

[bveeramani]

Description

If you try to iterate through a dataset, you get an unhelpful error.

>>> for x in ds:
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'Dataset' object is not iterable

We should provide a more descriptive error along the lines of:

TypeError: You're trying to iterate through a dataset, but this isn't supported. If you're trying to view records, call `Dataset.take`. If you're trying to transform records, call `Dataset.map_batches`.

Use case

New users may intuitively try to iterate through a dataset. The error message should redirect users to the appropriate methods.