Exception raised when a Tempo value cannot be rendered as an ISO 8601
string because it contains a construct with no ISO 8601 representation.
The only such construct at present is a nearest-weekday recurrence —
the cron W day-of-month modifier (15W, LW), parsed by Tempo.Cron
into a :nearest_weekday selection token. Like RFC 5545 BYSETPOS/WKST
it has no ISO 8601 designator; unlike those, no project-specific
designator was minted for it (the equivalent day-level operation is
Tempo.nearest_working_day/2). Such a value round-trips only through its
cron string, not through Tempo.to_iso8601/1.