remove redundant code and better error msg

Signed-off-by: allencloud <allen.sun@daocloud.io>

client/swarm_unlock.go

@@ -8,10 +8,6 @@ import (
 // SwarmUnlock unlockes locked swarm.
 func (cli *Client) SwarmUnlock(ctx context.Context, req swarm.UnlockRequest) error {
 	serverResp, err := cli.post(ctx, "/swarm/unlock", nil, req, nil)
-	if err != nil {
-		return err
-	}
-
 	ensureReaderClosed(serverResp)
 	return err
 }

client/volume_prune.go

@@ -29,7 +29,7 @@ func (cli *Client) VolumesPrune(ctx context.Context, pruneFilters filters.Args)
 	defer ensureReaderClosed(serverResp)
 
 	if err := json.NewDecoder(serverResp.body).Decode(&report); err != nil {
-		return report, fmt.Errorf("Error retrieving disk usage: %v", err)
+		return report, fmt.Errorf("Error retrieving volume prune report:: %v", err)
 	}
 
 	return report, nil