Removed unnecessary failure handling in agent HTTP API handlers.
Removed unnecessary failure handling in agent HTTP API handlers.

The current agent HTTP API handlers either unnecessarily handle failures
on always-ready futures, or return "500 Internal Server Error"
unnecessarily. This patch removes those unnecessarily code.

Review: https://reviews.apache.org/r/68755

reneploetz/mesos,gsantovena/mesos,dforsyth/mesos,jpeach/mesos,reneploetz/mesos,kaysoky/mesos,jpeach/mesos,jpeach/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,dforsyth/mesos,chhsia0/mesos,dforsyth/mesos,chhsia0/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,chhsia0/mesos,abudnik/mesos,abudnik/mesos,abudnik/mesos,gsantovena/mesos,jpeach/mesos,chhsia0/mesos,dforsyth/mesos,kaysoky/mesos,gsantovena/mesos,dforsyth/mesos,reneploetz/mesos,reneploetz/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,chhsia0/mesos,Gilbert88/mesos,kaysoky/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,chhsia0/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos