Obscure error when _describe() is not implemented properly

[noklam]

Description

Related #4024

Context

unused_dataset:
  type: demo_project.datasets.NokDataset
  # filepath: data/01_raw/companies.csv

class NokDataset(AbstractDataset):

    def _load(self):
        return 1

    def _save(self,data):
        ...

    def _describe(self):
        ...

Arguably I have a bad implementation here as _describe is not returning dict properly. It takes me a while to find out this is due to a bad dataset implementation as I was not printing a dataset but simply running kedro ipython. This would work prior 0.19.7 because _describe was never used.

The error message is not helpful as a combination of

1. it doesn't tell me which dataset is causing the problem
2. unexpected __repr__ call when I am not printing.