helper: Fix error message — heartwood

Git was outputting a confusing message when an unuspported option was used:

fatal: helper rad does not support ‘push-option’

This fixes it.

 error: refusing to delete default branch ref 'refs/heads/master'
 error: failed to push some refs to 'rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi'
 ```
 If you pass an unsupported push option, you get an error:
 ``` (stderr) (fail)
 $ git push -o alien rad HEAD:alice/2
 error: unknown push option "alien"
 ```

                 println!("ok");
             }
             ["option", "push-option", args @ ..] => {
                 if push_option(args, &mut opts).is_ok() {
                     println!("ok");
                 } else {
                     println!("unsupported");
                 }
                 // Nb. Git documentation says that we can print `error <msg>` or `unsupported`
                 // for options that are not supported, but this results in Git saying that
                 // "push-option" itself is an unsupported option, which is not helpful or correct.
                 // Hence, we just exit with an error in this case.
                 push_option(args, &mut opts)?;
                 println!("ok");
             }
             ["option", "progress", ..] => {
                 println!("unsupported");