Fixed the docker executor to handle inspect failures.
Fixed the docker executor to handle inspect failures.

Review: https://reviews.apache.org/r/49674

gsantovena/mesos,verizonlabs/mesos,zmalik/mesos,dforsyth/mesos,zmalik/mesos,zmalik/mesos,craimbert/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,zmalik/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,craimbert/mesos,chhsia0/mesos,jpeach/mesos,neilconway/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,neilconway/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,yuquanshan/customizedMesos,craimbert/mesos,gsantovena/mesos,gsantovena/mesos,abudnik/mesos,asamerh4/mesos,jpeach/mesos,neilconway/mesos,craimbert/mesos,neilconway/mesos,zmalik/mesos,dforsyth/mesos,dforsyth/mesos,asamerh4/mesos,chhsia0/mesos,yuquanshan/customizedMesos,andschwa/mesos,verizonlabs/mesos,craimbert/mesos,gsantovena/mesos,neilconway/mesos,jpeach/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,yuquanshan/customizedMesos,andschwa/mesos,jpeach/mesos,verizonlabs/mesos,Gilbert88/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,neilconway/mesos,gsantovena/mesos,andschwa/mesos,asamerh4/mesos,zmalik/mesos,gsantovena/mesos,abudnik/mesos,gsantovena/mesos,neilconway/mesos,kaysoky/mesos,chhsia0/mesos,verizonlabs/mesos,verizonlabs/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,jpeach/mesos,asamerh4/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,kaysoky/mesos,asamerh4/mesos,chhsia0/mesos,verizonlabs/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,abudnik/mesos,craimbert/mesos,andschwa/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,andschwa/mesos,zmalik/mesos,chhsia0/mesos,craimbert/mesos,Gilbert88/mesos,kaysoky/mesos,asamerh4/mesos,reneploetz/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,verizonlabs/mesos,dforsyth/mesos,andschwa/mesos,yuquanshan/customizedMesos,jpeach/mesos,asamerh4/mesos