Function map_error 

pub fn map_error<T, E1, E2, F>(
    src: Source<Result<T, E1>>,
    f: F,
) -> Source<Result<T, E2>>
where
    T: Send + 'static,
    E1: Send + 'static,
    E2: Send + 'static,
    F: FnMut(E1) -> E2 + Send + 'static,

Map the error variant via f. Both Ok and Err continue downstream; only the Err payload type changes.

Akka.NET: Source.SelectError.