Add script_version suggestion when dataset/metric not found
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Adds a helpful prompt to the error message when a dataset/metric is not found, suggesting the user might need to pass `script_version="master"` if the dataset was added recently. The whole error looks like:

> Couldn't find file locally at blah/blah.py, or remotely at https://raw.githubusercontent.com/huggingface/datasets/1.1/metrics/blah/blah.py or https://s3.amazonaws.com/datasets.huggingface.co/datasets/met
rics/blah/blah.py.
If the dataset was added recently, you may need to to pass script_version="master" to find the loading script on the master branch.
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