Transformer: Ignore missing index hits

I've been having some trouble with a very large index where very occasionally, a record that still exists inside elastic search no longer exists in the local database.

I'm yet to track down which process in the application is causing this but because of how large the table is (20M rows) and how long it takes to index, I feel there is scope for this PR to ignore the fact that not all objects were returned from the transformer.

Im also looking into writing some maintenance commands that will incrementally validate/reindex records as a complete reindex is not feasible with a large dataset.