Better cached path
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I should have read this PR before doing my own: https://github.com/huggingface/nlp/pull/62 :D 
will close mine. Looks great :-)  > Awesome, this is really nice!
> 
> By the way, we should improve the `cached_path` method of the `transformers` repo similarly, don't you think (@patrickvonplaten in particular).

Yeah, we should do the same in `transformers` I think - will note it down.
### Changes:
- The `cached_path` no longer returns None if the file is missing/the url doesn't work. Instead, it can raise `FileNotFoundError` (missing file), `ConnectionError` (no cache and unreachable url) or `ValueError` (parsing error)
- Fix requests to firebase API that doesn't handle HEAD requests...
- Allow custom download in datasets script: it allows to use `tf.io.gfile.copy` for example, to download from google storage. I added an example: the `boolq` script
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