e33181e883 stopped caching every None from _cron_interval_minutes, so a
malformed cron expression (croniter loaded, expression invalid) was
re-parsed on every in-flight-guard check. Track whether croniter actually
loaded and cache unless the None came from a failed import: a bad-expr
None is stable, an ImportError None is transient. The kept #127182
regression test now asserts the import-failure None stays uncached.
Also drop the redundant recovery_kind operand in
_recover_missing_next_run (_is_recoverable_error_job already requires a
cron/interval schedule) and tighten two comments.
8a1efff136