Added needed forward-declaration for InternalServerError.
Added needed forward-declaration for InternalServerError.

As currently written the code relies on `process::http` being the only
visible namespace with name `http`, an assumption which will break as
soon as we introduce declarations from `mesos::http`.

Fix this as customary by explicitly pulling the needed name into the
local name lookup scope.

Review: https://reviews.apache.org/r/46084/

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