Macro defensive

macro_rules! defensive {
    () => { ... };
    ($error:expr $(,)?) => { ... };
    ($error:expr, $proof:expr $(,)?) => { ... };
}

Other helper macros from frame_support that help with asserting in tests. Generic function to mark an execution path as ONLY defensive.

Similar to mark a match arm or if/else branch as unreachable!.