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/api/command_spec.lua

@@ -828,7 +828,7 @@ describe('nvim_create_user_command', function()
       })
     ]])
     feed(':Test <Tab>')
-    eq('E5108: Lua function: [NULL]', api.nvim_get_vvar('errmsg'))
+    eq('E5108: Lua function: nil', api.nvim_get_vvar('errmsg'))
     eq('Test ', fn.getcmdline())
     assert_alive()
   end)