"The dataset does not exist" error
From [Slack](https://huggingface.slack.com/archives/CUTRZ7YJ0/p1652995016789019)

> Hello, I’m facing an issue with the dataset preview feature. I uploaded a dataset here: https://huggingface.co/datasets/allenai/wmt22_african, and added a loading script. I have also confirmed that loading the dataset works with `streaming=True`. However, the dataset preview’s error message says that “The dataset does not exist.“. (edited) 
>
> It looks like the dataset viewer is now visible. Since I didn’t change anything, I’m assuming it was fixed on your end. Thanks! 

<img width="1128" alt="Capture d’écran 2022-05-20 à 09 04 05" src="https://user-images.githubusercontent.com/1676121/169472265-4b818b7b-2d09-4065-9a54-088ae0d8e7f2.png">

We should improve the different states, and not show this kind of error message if it's obvious that the dataset exists. Error messages should be reserved for errors with the dataset script, not for normal states (cache not filled yet)
"The dataset does not exist" error: From [Slack](https://huggingface.slack.com/archives/CUTRZ7YJ0/p1652995016789019)

> Hello, I’m facing an issue with the dataset preview feature. I uploaded a dataset here: https://huggingface.co/datasets/allenai/wmt22_african, and added a loading script. I have also confirmed that loading the dataset works with `streaming=True`. However, the dataset preview’s error message says that “The dataset does not exist.“. (edited) 
>
> It looks like the dataset viewer is now visible. Since I didn’t change anything, I’m assuming it was fixed on your end. Thanks! 

<img width="1128" alt="Capture d’écran 2022-05-20 à 09 04 05" src="https://user-images.githubusercontent.com/1676121/169472265-4b818b7b-2d09-4065-9a54-088ae0d8e7f2.png">

We should improve the different states, and not show this kind of error message if it's obvious that the dataset exists. Error messages should be reserved for errors with the dataset script, not for normal states (cache not filled yet)