Change API response when requesting an invalid event ID

Currently, if you request a single event by ID (e.g. /events.php?id=99999) and the ID is non-existent or the event was deleted, we return a 200 OK response with an empty array. This is essentially a "not found" response, but in a roundabout way. We should just return a proper 404 response.

(But we should continue to return an empty array for day range requests that don't have any events scheduled. An empty array is meaningful in that scenario.)