LibCore+Everywhere: Return ErrorOr from ConfigFile::sync()

Currently this method always succeeds, but that won't be true once we
switch to the Core::Stream API. :^)

Some of these places would ideally show an error message to the user,
since failure to save a file is significant, but let's not get
distracted right now.

Userland/Utilities/ini.cpp

@@ -35,7 +35,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     if (value_to_write) {
         config->write_entry(group, key, value_to_write);
-        config->sync();
+        TRY(config->sync());
         return 0;
     }