make example_vignette.png optional also for defautl page

build.py

@@ -133,6 +133,7 @@ if __name__ == "__main__":
             content = content.replace("questions.html", lang + "/questions.html")
             content = content.replace("strings.js", lang + "/strings.js")
             content = content.replace("./logo_g.png", lang + "/logo_g.png")
+            if os.path.isfile(source_lang_dir + "/example_vignette.png"):
                 shutil.copy(source_lang_dir + "/example_vignette.png", build_lang_dir )
             # overwrite file
             with open(target, "w") as f: