Macro err 

macro_rules! err {
    ($variant:ident) => { ... };
    ($variant:ident, $message:expr) => { ... };
    ($variant:ident, $template:expr, $($args:expr),+) => { ... };
    (@new $variant:ident, $message:expr) => { ... };
    (@backtrace) => { ... };
}

Create an instance of Error