Handle failure during Booking creation

Booker API gives a conflict failure if an existing booking exists in the room at an overlapping time:

ERROR:essmsync.utils:Request https://booker-api-staging.azurewebsites.net/api/booker/booking Failed: 409 Client Error: Conflict for url: https://booker-api-staging.azurewebsites.net/api/booker/booking

Typically, an update needs to happen before a creation but still the order of updates cannot be guaranteed to complete.

Improve handling of booking creation/update failures. Also need to cancel events that have been deleted from TermTime.