Function err 

pub fn err<T, E, M>(inner: M) -> impl Matcher<Result<T, E>>
where
    M: Matcher<E>,
    T: Debug,

Matches an Err whose contained value satisfies inner.

use test_better_core::TestResult;
use test_better_matchers::{eq, err, check};

fn main() -> TestResult {
    check!(Err::<i32, &str>("boom")).satisfies(err(eq("boom")))?;
    check!(Ok::<i32, &str>(0)).violates(err(eq("boom")))?;
    Ok(())
}