Improve error message for missing pacakges from inside dataset script

[mariosasko]

Improve the error message for missing packages from inside a dataset script:

With this change, the error message for missing packages for bigbench looks as follows:

ImportError: To be able to use bigbench, you need to install the following dependencies:
        - 'bigbench' using 'pip install "bigbench @ https://storage.googleapis.com/public_research_data/bigbench/bigbench-0.0.1.tar.gz"'

And this is how it looked before:

ImportError: To be able to use bigbench, you need to install the following dependencies['bigbench', 'bigbench', 'bigbench', 'bigbench'] using 'pip install "bigbench @ https://storage.googleapis.com/public_research_data/bigbench/bigbench-0.0.1.tar.gz" bigbench bigbench bigbench' for instance'

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[lhoestq]

I opened a PR before I noticed yours ^^' You can find it here: #4484

The only comment I have regarding your message is that it possibly shows several pip install commands, whereas one can run one single pip install command with the list of missing dependencies, which is maybe simpler.

Let me know which one your prefer

[mariosasko]

Closing in favor of #4484.