Macro relm::connect_async

macro_rules! connect_async {
    ($object:expr, $async_method:ident, $relm:expr, $msg:expr) => { ... };
    ($object:expr, $async_method:ident ( $($args:expr),* ), $relm:expr, $msg:expr) => { ... };
    ($object:expr, $async_method:ident, $relm:expr, $msg:expr, $fail_msg:expr) => { ... };
    ($object:expr, $async_method:ident ( $($args:expr),* ), $relm:expr, $msg:expr, $fail_msg:expr) => { ... };
}

Connect an asynchronous method call to send a message. The variants with $fail_msg will send this message when there's an error. Those without this argument will ignore the error.