worker: improved error message on generateChunks
worker: improved error message on generateChunks

Issue #3407
Error messages did not specify the tablet or table they were
failing on.

alainjobart/vitess,mattharden/vitess,tinyspeck/vitess,alainjobart/vitess,vitessio/vitess,dcadevil/vitess,vitessio/vitess,sougou/vitess,mattharden/vitess,mahak/vitess,tirsen/vitess,alainjobart/vitess,HubSpot/vitess,alainjobart/vitess,mahak/vitess,sougou/vitess,tinyspeck/vitess,mattharden/vitess,vitessio/vitess,tirsen/vitess,sougou/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,mattharden/vitess,HubSpot/vitess,davygeek/vitess,davygeek/vitess,mattharden/vitess,HubSpot/vitess,mattharden/vitess,mattharden/vitess,HubSpot/vitess,alainjobart/vitess,mahak/vitess,enisoc/vitess,tirsen/vitess,tirsen/vitess,tirsen/vitess,tinyspeck/vitess,sougou/vitess,tirsen/vitess,dcadevil/vitess,davygeek/vitess,dcadevil/vitess,tirsen/vitess,alainjobart/vitess,dcadevil/vitess,sougou/vitess,mattharden/vitess,mattharden/vitess,dcadevil/vitess,enisoc/vitess,vitessio/vitess,davygeek/vitess,sougou/vitess,mattharden/vitess,HubSpot/vitess,HubSpot/vitess,vitessio/vitess,tirsen/vitess,sougou/vitess,mahak/vitess,tirsen/vitess,sougou/vitess,mahak/vitess,tinyspeck/vitess,HubSpot/vitess,davygeek/vitess,enisoc/vitess,mahak/vitess,alainjobart/vitess,enisoc/vitess,davygeek/vitess,vitessio/vitess,enisoc/vitess,davygeek/vitess,enisoc/vitess,enisoc/vitess,tinyspeck/vitess,tinyspeck/vitess,dcadevil/vitess,mahak/vitess,HubSpot/vitess,alainjobart/vitess,dcadevil/vitess