Function check 

pub fn check(
    doc_dir: Option<PathBuf>,
    site_dir: Option<PathBuf>,
) -> Result<(), Vec<Error>>

Runs the three checks, reporting every failure rather than the first.

Errors

One Error per broken link, plus one per missing input. Collected rather than short-circuited: a developer who has just moved a module wants the whole list, and a checker that stops at the first makes the second look like a new regression next time.