Currently, in the code base, we have one distinct place where a sub-level error code would be worth returning back. Look here:

            WT_RET_MSG(session, EBUSY,
              "checkpoint %s blocked by hot backup: it would delete an existing named checkpoint, "
              "and such checkpoints cannot be deleted during a hot backup",
              ckpt->name);

We should convert this WT_RET_MSG into using sub-level error code and file a server ticket for MongoDB to deal with it.