Let's make this a require() since it should always be performed at runtime. We also don't support using disk, memory, or deserialized when offheap is set, so I think a fuller check would look something like

```
if (useOffheap) {
  require(useDisk == false, "Off-heap storage level does not support using disk")
  require(useMemory == false, "Off-heap storage level does not support using heap memory")
  require(deserialized == false, "Off-heap storage level does not support deserialized storage")
  require(replication == 1, "Off-heap storage level does not support multiple replication")
}
```

and we can remove some of these in the future when they're actually supported.