Export region in diary file to iCalendar format.
All diary entries in the region from MIN to MAX in the current buffer are
converted to iCalendar format. The result is appended to the file
This function attempts to return t if something goes wrong. In this
case an error string which describes all the errors and problems is
written into the buffer `*icalendar-errors*'.
(fn MIN MAX ICAL-FILENAME)