materialize-dynamodb: log failed batch writes in stores

Log out information about the batch attempting to be stored if the store
operation fails.

materialize-dynamodb/transactor.go

@@ -301,6 +301,7 @@ func (t *transactor) storeWorker(ctx context.Context, batches <-chan map[string]
 					RequestItems: batch,
 				})
 				if err != nil {
+					log.WithField("batch", batch).Error("failed batch write")
 					return err
 				}