Catch some conflict check errors more gracefully
In the AdminUI, if you schedule a recurrent event with the start & end date on the same day, not the same day of week as the recurrent event, you get an appropriate UI error (Scheduling error: The event has already ended), but also this long stack trace in the logs that's not really helpful.
POST to /admin-ng/event/new/conflicts with metadata like:
and gets an http 500 internal server error back.
2019-02-16 17:39:31,597 | ERROR | qtp1466124335-1047 | (AbstractEventEndpoint:2235) - Unable to find conflicting events for jdaylt2, 2019-02-16T02:00:00Z, 2019-02-16T02:55:00Z: java.lang.IllegalArgumentException: Requirement 'periods' must hold true
This change avoids back-end errors, but leads to the Admin UI thinking an event has been created when none has (because the range is empty). Needs some sanity-checking on the Admin UI, or a slightly different response from the conflict check that means "no events in range" rather than "no conflicts".
Also need to check how the external API handles this.