Function publish_through 

pub fn publish_through(
    dir: &Path,
    stem: &str,
    outcome: &Result<EmittedDocument, Verdict>,
    write: &mut dyn FnMut(&mut dyn Write, &[u8]) -> Result<()>,
) -> Result<Published>

publish, writing every file’s bytes through write — the seam a test uses to make a write fail half way.