The condition added here is to guard against WT snapshot isolation with read timestamp semantics where a read timestamp of T can sometimes not see committed updates [index build completions] at time < T if the WT transaction were opened before the update [index build] was committed.

However, while it's possible the timing of events would show storage seeing an index as building, but not complete, it's also possible the storage engine doesn't see the index build having begun. In that circumstance, a call to `_catalogIsReady` is fatal.