Macro hopper_error 

macro_rules! hopper_error {
    (
        base = $base:literal;
        $( $name:ident ),+ $(,)?
    ) => { ... };
}

Generate sequential error codes.

hopper_error! {
    base = 6000;
    Undercollateralized,  // 6000
    Expired,              // 6001
    InvalidOracle,        // 6002
}