Get error when require module that doesn't exist

`require 'doesnotexist'` should produce an error.

I come from the discussion https://github.com/LuaLS/lua-language-server/discussions/2339

I don't find any related issues so opening a new one.

Start by reading discussion 2339, then locate the Lua language server's handling of `require` calls. Reproduce `require 'doesnotexist'` and trace whether missing modules are reported. Done means this unresolved module produces an error diagnostic.

Written by the indexing model from the issue text.