InvalidRollForward exception should be unreachable

While reviewing PR input-output-hk/ouroboros-network#773 for Issue input-output-hk/ouroboros-network#231, @nc6 observed and I agreed that a `ForkTooDeep` exception should be raised prior to the mini protocol reaching a state where `InvalidRollForward` could be raised. PR input-output-hk/ouroboros-network#773 introduces `InvalidRollForward`, because otherwise the `RealPBFT` tests fail. We're merging with this Issue as a follow-up obligation.