Updated the HTTP result returned by failures of authn/authz.
Updated the HTTP result returned by failures of authn/authz.

Changes authentication and authorization happening on the libprocess
level to be in line with failures possibly returned by Mesos
authorization as currently implemented by the HTTPProxy::process
function. The HTTP result returned on failures has changed from
InternalServerError (500) towards ServiceNotAvailable (503) and now
contains a message describing the problem.

Review: https://reviews.apache.org/r/48920/

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