The router role/mongos rewrites certain state-change errors before it returns them to clients, to prevent errors that mongos is merely 'proxying' from underlying shard-servers from confusing drivers about the state of mongos itself. Today, that behavior is gated by an explicit check on process being exclusively a router. However, the router-service of a process that provides both router and shard services should do the same rewrites for the same reasons. So we should remove the explicit check