ical-recurrence: test that an hourly recurrence rule does not crash
recurrenceRuleDateTimeOccurrences ends in a catch-all arm that calls
error, so any calendar containing one of the three sub-daily
frequencies throws an ErrorCall from pure code. A caller cannot catch
that through Conform, which is the whole point of recurring inside it.
Hourly, minutely and secondly are legal RFC 5545 frequencies, so this
should either recur as asserted here or, until it is implemented, fail
as an unfixable RecurrenceError. The catch-all also hides the gap from
the exhaustiveness checker, so adding a frequency would not point here.
The four pending examples in ICal.Recurrence.RecurrenceRuleSpec already
document the missing feature; this one is about the failure mode.