Update/remove of nonexistent doc shouldn't add an item to transaction

[mattmccutchen]

If update is called with the ID of a document that doesn't exist, it logs a message but goes on and adds an item for the nonexistent document to the transaction anyway, which might have unexpected consequences later. I'm guessing remove does the same thing, but I haven't tested it. I'd expect these operations not to add an item.

(#80 would subsume this issue.)