[CalDAV] Invert success value of schedule outbox free_busy

When a participant is added to a meeting, an RFC6638 outbox-based freebusy query is performed.

Unfortunately, the boolean success value returned from the function (ecb_caldav_get_free_busy_from_schedule_outbox_sync) which performs this query is inverted! Thus when the query is successful ecb_caldav_get_free_busy_sync sees it as unsuccessful and falls back to ecb_caldav_get_free_busy_from_principal_sync (which is how I encountered the bug fixed in !7 (closed)).

This patch inverts the return value to fix this behaviour.