gpt-auto-generator: don't eat up errors of generator_enable_remount_fs_service()

I cannot see a reason why we should ignore this error, so let's not. We
use RET_GATHER() on the returns anyway, i.e. collect errors but
continue, so it makes sense to collect this one too.

src/gpt-auto-generator/gpt-auto-generator.c

@@ -648,7 +648,9 @@ static int add_partition_root_rw(DissectedPartition *p) {
                 return 0;
         }
 
-        (void) generator_enable_remount_fs_service(arg_dest);
+        r = generator_enable_remount_fs_service(arg_dest);
+        if (r < 0)
+                return r;
 
         path = strjoina(arg_dest, "/systemd-remount-fs.service.d/50-remount-rw.conf");