vtcompose: add err check
vtcompose: add err check

Signed-off-by: Zhou Hao <f577ade201aa03d612c19915a089dc6084d45cd9@cn.fujitsu.com>

mahak/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess,tinyspeck/vitess,mahak/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess