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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I have the same question (in Python). I raised an exception and it ends up being uncaught which causes the tests to exit with error code 1, so they fail even though all the tests show passed. It looks like many people have completed this kata though so maybe we're missing something obvious. Though I think the instructions should be more clear about precisely what "throw an error" means.