JS API exposes un-previewed tables to the UI in some cases, causing errors on subscribe

[niloc132]

#188 should make this irrelevant, by letting us apply preview as needed as part of creating the subscription.

dh.WidgetExportedObject.fetch() and dh.PartitionedTable.getTable() both will return promises of tables but not apply any previewing. If the UI attempts to subscribe to those columns, there will be an error, as the current JS barrage implementation cannot handle most types of arrays.