Function force_error_in 

pub fn force_error_in(
    run_dir: &Path,
    message: &str,
    now: i64,
) -> ForceCancelOutcome

Force the run in run_dir to Error with message, stamping updated_at.

For the spawn that never became a run. The spawner stakes out the run directory and writes a Starting placeholder before building the agent, so a spawn that fails leaves something to diagnose - but Starting is not terminal, so that placeholder went on claiming the run was alive for ever, showing up in lev ps and the dashboard with nothing behind it (issue #190). Recording the failure where the placeholder is turns it into an answer.