Fix misleading error message in `_assert_is_pandas_df`

[nathanjmcdougall]

This helper function _assert_is_pandas_df is called for a variety of file formats, but the error message mentions CSV specifically:

pins-python/pins/drivers.py

Lines 17 to 23 in 70380b4

	def _assert_is_pandas_df(x):
	import pandas as pd
	if not isinstance(x, pd.DataFrame):
	raise NotImplementedError(
	"Currently only pandas.DataFrame can be saved to a CSV."
	)

I think this function should be changed to accept a type argument so that an appropriate message can be constructed dynamically.

[isabelizimm]

That makes sense to use a type argument! It is a helper function that is only used in pins-python/pins/drivers.py, and always is in the context of an if type == ... block 👍