Sync problem specs

[BNAndras]

Here are the results of running ./bin/configlet sync --docs --metadata --tests include -uy. The two new largest-series-product tests just change the expected error messages for those two situations, but the track expects Nothing returned, so there's no need to update the tests themselves.

[jiegillet]

Thank you, looks good.
I must ask, should we consider changing largest-series-product so that it returns errors instead of a maybe?