Turns out there's no danger involved in missing the marks in the slingshot maneuver. If it happened they have a "free return" trajectory planned and have enough food water and air left in the service module for up to ten additional days.
The danger though is, if they skip off Earth's atmosphere. This would happen only after the service module is jettisoned as part of the return approach, so they won't have enough air to survive free return from there and might even wind up in orbit permanently.