The aggregate commands called by agg_lookup.js may throw a QueryPlanKilled error when the balancer is enabled due to range deletions.

Before SPM-3964, range deletions caused secondary queries to overlook documents. But after SPM-3964, secondary queries targeting sharded collections may fail with QueryPlanKilled error due to range deletions happening in the background.

Therefore, we should ignore the QueryPlanKilled error when the balancer is enabled.