Function preflight 

pub fn preflight(
    plan: &Plan,
    challenge: Challenge,
    uid: &str,
) -> Result<(), String>

Everything that must be true before the first privileged byte.

Checked in one pass and reported together: a tool that fails on the first missing thing, is fixed, then fails on the second is a tool that gets run four times, and each of those runs asked for root.

Errors

Returns every unmet precondition, one per line.