helper: Fix error message

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

  fatal: helper rad does not support 'push-option'

This fixes it.

radicle-cli/examples/git/git-push.md

@@ -69,3 +69,9 @@ $ git push rad :master
 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"
+```

radicle-remote-helper/src/lib.rs

@@ -146,11 +146,12 @@ pub fn run(profile: radicle::Profile) -> Result<(), Error> {
                 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");