No error when flushing DataAccess that was not created, but cannot load it either

In `RAMIntDataAccess.java`:

```java
@Test
public void flushWithoutCreate() {
DataAccess da = createDataAccess("abc");
da.setHeader(0, 6);
assertEquals(6, da.getHeader(0));
// no error here...
da.flush();
da.close();

DataAccess loaded = createDataAccess("abc");
// this will fail because we did not call create() before flush()
assertTrue(loaded.loadExisting());
}
```

Should we throw an error when trying to `flush` before calling `create`? Or should it just be possible to do this and we have to call `create` after `loadExisting`?