Update instructions.md for Lua error handling

[RitchieFlick]

The instructions did not mention that the use of the error keyword is required. In addition, following the Concepts up to this exercise did not mention the error keyword, as such additional instructions have been added.

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Edit: https://forum.exercism.org/t/mentioning-of-the-error-keyword/12686

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[glennj]

Or assert() -- https://www.lua.org/manual/5.4/manual.html#pdf-assert

[RitchieFlick]

Or assert() -- https://www.lua.org/manual/5.4/manual.html#pdf-assert

I tried it and that works as well for the exercise, so I added this to the description.