I think this points to a bug in hydra-head.  Since hydra-head now raises `CanCan::AccessDenied` instead of `Hydra::AccessDenied`, should we make it also rescue `CanCan::AccessDenied` here? https://github.com/projecthydra/hydra-head/blob/3cace8778a8deb80a5dbcb8d949778604bc48aec/hydra-core/app/controllers/concerns/hydra/controller/controller_behavior.rb#L19