DAV resource: support exception incidences

My calendar has a few events like
[...]
BEGIN:VEVENT   # the only one
DTSTAMP:20210510T164410Z
CREATED:20210510T164409Z
LAST-MODIFIED:20210510T164410Z
DTSTART;TZID=Europe/Paris:20210511T143000
DTEND;TZID=Europe/Paris:20210511T153000
RECURRENCE-ID;TZID=Europe/Paris:20210513T143000
[...]

They show up in roundcube but were missing in korganizer.
I'm no ICAL expert but I think those are exceptions
to a recurring event, detached from that recurring event.

And indeed after this fix, they show up as "Recurrence: Exception"
in korganizer.

Found by debugging "Failed to parse item data" warnings from
the DAV resource.