test: Use a better error message when code.lua doesn't exist (#1722)

data/core/about.cfg

@@ -1286,6 +1286,10 @@
     [entry]
         name = "Luiz Fernando de Faria Pereira (lfernando)"
     [/entry]
+    [entry]
+        name = "Luther Thompson"
+        email = "lutheroto_AT_gmail.com"
+    [/entry]
     [entry]
         name = "lv-zheng"
     [/entry]

data/scenario-test.cfg

@@ -1397,7 +1397,19 @@ My best advancement costs $next_cost gold and I’m $experience|% there."
             # The file is not in the repository to allow adding custom stuff to it.
             [command]
                 [lua]
-                    code=<< wesnoth.dofile("lua/code.lua").main() >>
+                    code=<<
+                        local codeFile = "lua/code.lua"
+                        if wesnoth.have_file(codeFile) then
+                            local code = wesnoth.dofile(codeFile)
+                            if type(code) == "table" and type(code.main) == "function" then
+                                code.main()
+                            else
+                                wesnoth.message(codeFile.." must return a table with a 'main' function.")
+                            end
+                        else
+                            wesnoth.message("To use this command, create a file 'data/lua/code.lua' in the wesnoth source directory that returns a table with a 'main' function.")
+                        end
+                    >>
                 [/lua]
             [/command]
         [/set_menu_item]