[CAL-2578] Fix all unhandled errors in handleNewBooking flow.

[hariombalhara]

Fix all unhandled errors in handleNewBooking flow. Those errors cause a 500 and instead we should server a proper status code with a message.

e.g. When a user isn't available for a timeslot we throw error 'Some users are unavailable for booking', it should have been `throw new HttpError({statusCode: 400})

_{From SyncLinear.com | CAL-2578}