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
 }