Macro btlib::btensure

macro_rules! btensure {
    ($cond:expr, $msg:literal $(,)?) => { ... };
    ($cond:expr, $err:expr $(,)?) => { ... };
    ($cond:expr, $fmt:expr, $($arg:tt)*) => { ... };
}

Ensures that an expression evaluates to true, and if it does’nt, returns an error.