After creating an IsoUnion schema, inserting a tuple into a relation violating the predicate should throw a constraint error

```
employee:=relation{tuple{name "Steve", boss ""}, tuple{name "Cindy", boss "Steve"}, tuple{name "Sam", boss "Steve"}}
:addschema splitboss (isounion "boss" "peon" "employee" boss="", isopassthrough "true", isopassthrough "false")
:setschema splitboss
insert peon relation{tuple{name "Elvis", boss ""}}
```
results in silent success and a discarded tuple which leaks the fact that this is a schema. A normal relvar would throw a constraint violation error instead which is what should happen in the schema context as well.