Macro assert_err 

macro_rules! assert_err {
    ($result:expr) => { ... };
    ($result:expr, $msg:expr) => { ... };
}

Assert that a result is an error with detailed message

Provides better error messages when testing error cases.

Example

use chicago_tdd_tools::assert_err;

let result: Result<u32, String> = Err("error".to_string());
assert_err!(result);

// With custom message
let result2: Result<u32, String> = Err("error".to_string());
assert_err!(result2, "Expected error case");