Fixed bug in fetcher.cpp when HDFS client is not available.
Fixed bug in fetcher.cpp when HDFS client is not available.

When trying to download an artifact with the Hadoop client,
and the client is not `available()` we correctly return a
false value, but then in the error message, we tried to
make a call to `Try::error` which failed and crashed Master.

This fixes it.

Review: https://reviews.apache.org/r/37584

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