Refactor: Raise exceptions, rather than using a deep `quit`

This isn't just a Nim thing.

Background:
https://forum.nim-lang.org/t/4042

As an example, the below code doesn't echo:
```Nim
try:
quit(0)
finally:
echo "hi"
```

(Try in the Nim playground [here](https://play.nim-lang.org/#ix=2M5W)).

For one better design, see [`check_uuids.nim`](https://github.com/exercism/nim/blob/be1382016edd67eb04f35a4468286517135b6bd2/_test/check_uuids.nim#L349-L355) from the `exercism/nim` repo.