Macro describe_err::describing

macro_rules! describing {
    ($expr:expr) => { ... };
}

Wrap an error with an auto-generated description.

This macro is essentially a wrapper around describe. It expands to the following:

// let res = describing!(result_expression);
let res = result_expression.map_err(describe("result_expression"));

The returned Result can be pattern-matched or propagated as usual.