Update instructions.md to specify the error

[Gosev]

The instructions state that an error must be returned depending on the language, but here, the tests expect a -1 return (other Lua exercises expect a call to error(...))

[github-actions]

Hello. Thanks for opening a PR on Exercism. We are currently in a phase of our journey where we have paused community contributions to allow us to take a breather and redesign our community model. You can learn more in this blog post. As such, all issues and PRs in this repository are being automatically closed.

That doesn't mean we're not interested in your ideas, or that if you're stuck on something we don't want to help. The best place to discuss things is with our community on the Exercism Community Forum. You can use this link to copy this into a new topic there.

---

Note: If this PR has been pre-approved, please link back to this PR on the forum thread and a maintainer or staff member will reopen it.

[ryanplusplus]

Hey @Gosev, these instructions are generated from https://github.com/exercism/problem-specifications. The reason that the instructions are written generically is that they have to apply to all language tracks.