exercises(grains): assert, rather than returning error union

[ee7]

I think that returning an error union here isn't particularly interesting. I think most of the exercises that currently contain (or previously contained) a single-item error set are better if the relevant functions return an optional, or say that it is the caller's responsibility to do input validation.

This PR is a follow-up to https://github.com/exercism/zig/commit/290e7f31ea8a521752885c94c4e771398191dfae, which removed the negative input test case.

Note that we should be able to add the "bad input" tests again in the future - Zig plans to give std.testing the ability to expect a panic.

Merging this PR will break everybody's solution.

Refs: #229

Similar to #335

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To-do:

• [ ] Check whether we should add an .append file that explains how to assert