A manual cronjob(action='run') derived success from last_status == 'ok'
and read the error from last_error — so a run that now records
delivery_failed came back as success=False with error=None, an unexplained
failure. Surface last_delivery_error as the error in that case (the
#84006 direction, re-applied on the delivery_failed status), and pin the
manual-run completion summary to say 'Result: FAILED' over an undelivered
run. Document the status in the cron user guide.
Co-authored-by: webtecnica <webtecnica@gmail.com>