Support system closures

[musjj]

Occasionally, you need to create system closures that can fail. For example, when creating pointer callbacks in bevy_mod_picking:

On::<Pointer<Click>>::run(
    |event: Listener<Pointer<Click>>,
      ...|
     -> anyhow::Result<()> {
        ...
        Ok(())
    }
)

Maybe a macro syntax can be used for this:

sysfail!(|...| -> anyhow::Result<()> { ... })