3dt: add check for `ExecMainStatus`
3dt: add check for `ExecMainStatus`

make sure the main process returned 0 status code
https://github.com/dcos/3dt/pull/57
DCOS-9323

surdy/dcos,dcos/dcos,darkonie/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/mergebot-test-dcos,kensipe/dcos,mnaboka/dcos,mellenburg/dcos,vishnu2kmohan/dcos,dcos/dcos,darkonie/dcos,mesosphere-mergebot/dcos,darkonie/dcos,asridharan/dcos,BenWhitehead/dcos,vishnu2kmohan/dcos,mesosphere-mergebot/mergebot-test-dcos,GoelDeepak/dcos,dcos/dcos,mellenburg/dcos,xinxian0458/dcos,branden/dcos,BenWhitehead/dcos,surdy/dcos,GoelDeepak/dcos,kensipe/dcos,darkonie/dcos,mnaboka/dcos,asridharan/dcos,lingmann/dcos,dcos/dcos,darkonie/dcos,vishnu2kmohan/dcos,mesosphere-mergebot/dcos,amitaekbote/dcos,dcos/dcos,branden/dcos,jeid64/dcos,kensipe/dcos,mnaboka/dcos,xinxian0458/dcos,branden/dcos,amitaekbote/dcos,amitaekbote/dcos,mellenburg/dcos,BenWhitehead/dcos,surdy/dcos,surdy/dcos,mnaboka/dcos,BenWhitehead/dcos,jeid64/dcos,GoelDeepak/dcos,mesosphere-mergebot/dcos,kensipe/dcos,vishnu2kmohan/dcos,xinxian0458/dcos,asridharan/dcos,xinxian0458/dcos,mesosphere-mergebot/dcos,mnaboka/dcos,GoelDeepak/dcos,mesosphere-mergebot/mergebot-test-dcos,jeid64/dcos,asridharan/dcos,mellenburg/dcos,jeid64/dcos,lingmann/dcos,amitaekbote/dcos,lingmann/dcos,branden/dcos,lingmann/dcos