storage: make an error benign

This was showing up a lot in TestInitialPartitioning. If we're trying to
remove something but nothing needs to be removed, that seems OK (though
there is some question of why we're hitting this regularly).

Release note: None

pkg/storage/replicate_queue.go

@@ -501,8 +501,8 @@ func (rq *replicateQueue) findRemoveTarget(
 	}
 	if len(candidates) == 0 {
 		// If we timed out and still don't have any valid candidates, give up.
-		return roachpb.ReplicaDescriptor{}, "", errors.Errorf("no removable replicas from range that needs a removal: %s",
-			rangeRaftProgress(repl.RaftStatus(), existingReplicas))
+		return roachpb.ReplicaDescriptor{}, "", &benignError{errors.Errorf("no removable replicas from range that needs a removal: %s",
+			rangeRaftProgress(repl.RaftStatus(), existingReplicas))}
 	}
 
 	return rq.allocator.RemoveTarget(ctx, zone, candidates, existingReplicas)