nth-prime: Add missing test case and change function signature

[abo64]

Test case "there is no zeroth prime" is missing in our test suite.
And then it makes sense to change Prime.nth to return an Option[Int] instead of just Int.

While being at it: Perhaps also change all other test descriptions to be the same as in canonical-data.json?

On a side note: There are more exercises out of sync with x-common. How can we handle this? There is an issue on the Haskell track for this.

[ErikSchierboom]

I agree with all you suggestions.

As for how to make exercises sync, that's actually a hard problem. Maybe all tracks maintainers should automatically receive a notification when the canonical data of an exercise is changed?

[abo64]

Yes, that would be a good idea.
I would even say when the description.md changes.
And also when a new exercise is added.