Reduce severity or remove log message for PlanExecutor::DEAD error during find

this message is logged with 'error', but it's not clear it should rise to that level. For instance - it can happen if a PlanExecutor is killed because a collection is dropped while it was executing. In cases like this, nothing happened to corrupt server state or anything like that, so it's probably not worthy of a higher severity than log, and possibly even should be at a higher log level than 0.