fn inspect_err<F>(self, f: F) -> InspectErr<Self, F>
    where
        F: FnMut(&Self::Error),
        Self: Sized,
    {
        assert_stream::<Result<Self::Ok, Self::Error>, _>(InspectErr::new(self, f))
    }
Do something with the error value of this stream, afterwards passing it on.

This is similar to the `StreamExt::inspect` method where it allows
easily inspecting the error value as it passes through the stream, for
example to debug what's going on.
futures-util/src/stream/try_stream/mod.rs