Nicer exception
Nicer exception

If milestone_rc is None type, there was probably an issue in the
process that went silent (like the Github 404 issues). See
github3 lib for the None type retuns.

The non 404 issues are generally caught by
github3.exceptions.UnprocessableEntity , therefore we don't need
additional processing there.

cloudnull/rpc-openstack,git-harry/rpc-openstack,hughsaunders/rpc-openstack,BjoernT/rpc-openstack,prometheanfire/rpc-openstack,prometheanfire/rpc-openstack,cloudnull/rpc-openstack,robb-romans/rpc-openstack,cfarquhar/rpc-openstack,mancdaz/rpc-openstack,BjoernT/rpc-openstack,major/rpc-openstack,rcbops/rpc-openstack,jacobwagner/rpc-openstack,major/rpc-openstack,jacobwagner/rpc-openstack,mancdaz/rpc-openstack,galstrom21/rpc-openstack,rcbops/rpc-openstack,galstrom21/rpc-openstack,cfarquhar/rpc-openstack,hughsaunders/rpc-openstack,robb-romans/rpc-openstack,git-harry/rpc-openstack