v8_try macros for rust

v8_try.rs

macro_rules! v8_try {
  ($expr:expr, $args:ident) => ({
    let ret;
    match $expr {
      Ok(val) => {
        ret = Some(val);
      },
      Err(err) => {
        ret = None;
        $args.GetReturnValue().SetWithBool(false)
      }
    };
    ret.unwrap()
  })
}