fix(lua): use even safer and even better error() message conversion

problem:
this crashes neovim:

ondska = setmetatable({}, {__tostring = error})
error(ondska)

solution:

make it not crash neovim

test/functional/lua/luaeval_spec.lua

@@ -495,7 +495,7 @@ describe('luaeval()', function()
       remove_trace(pcall_err(command, [[call luaeval("error('ERROR')")]]))
     )
     eq(
-      'Vim(call):E5108: Lua: [NULL]',
+      'Vim(call):E5108: Lua: nil',
       remove_trace(pcall_err(command, [[call luaeval("error(nil)")]]))
     )
   end)