Update instructions.md for Lua error handling (#497)

* Update instructions.md for Lua error handling

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.

* Update instructions.md with assertion

Added an alternative way of using assertion.

exercises/practice/triangle/.docs/instructions.md

@@ -26,4 +26,13 @@ a + c ≥ b
 
 See [Triangle Inequality][triangle-inequality]
 
+### Error handling & Assertion
+
+Use error handling or assertion to check if the shape is a triangle at all:
+
+- [Error Keyword][error-keyword]
+- [Assertion][assertion]
+
 [triangle-inequality]: https://en.wikipedia.org/wiki/Triangle_inequality
+[error-keyword]: https://www.lua.org/manual/5.4/manual.html#pdf-error
+[assertion]: https://www.lua.org/manual/5.4/manual.html#pdf-assert