Scheduling: Scheduling an event currently in progress gives you the wrong error message

[gregorydlogan]

Scheduling an event which is currently in progress gives you an error that 'The event has already ended'.

Open questions:

• Is this something that's reasonable to support at all? Lots of CAs wouldn't check their calendars fast enough to catch many already-in-progress events, so this is probably something we can just refuse to allow via the UI?
• If we do want to support this, do we have an existing string or does it need a new one?

Noticed in testing #1069