Better ImportError message when a dataset script dependency is missing
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_The documentation is not available anymore as the PR was closed or merged._ Discussed offline with @mariosasko, merging :) Fwiw, i think this same issue is occurring on the datasets website page, where preview isn't available due to the `bigbench` import error For the preview of BigBench datasets, we're just waiting for bigbench to have a stable version on PyPI, instead of the one hosted on GCS ;)
When a depenency is missing for a dataset script, an ImportError message is shown, with a tip to install the missing dependencies. This message is not ideal at the moment: it may show duplicate dependencies, and is not very readable.

I improved it from
```
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'
```
to
```
ImportError: To be able to use bigbench, you need to install the following dependency: bigbench.
Please install it using 'pip install "bigbench @ https://storage.googleapis.com/public_research_data/bigbench/bigbench-0.0.1.tar.gz"' for instance'
```
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