Replace EpochInfo Identity by EpochInfo (Except PastHorizonException)

[edsko]

The Shelley ledger as well as the TPraos implementation require an EpochInfo Identity in their config. Such a thing is really an impossibility: the EpochInfo changes as the ledger changes. The hard fork combinator alleviates this problem to some extent by introducing the concept of a partial config, and constructing the right EpochInfo on the fly. However, the EpochInfo that we pass to the ledger and to TPraos nonetheless has a limited range, and any attempt to use it beyond that range will result in a pure exception. It would be much better if the EpochInfo we construct as type EpochInfo (Except PastHorizonException), so that an attempt to look past that horizon will result in an error we can more reliably catch.

The changes consensus side are trivial (in fact, we have to do extra work to turn these exceptions into pure exceptions); the more laborious part is updating the TPraos and Shelley ledger implementations.

That said, this is mostly to aid debugging: any attempt to look past the horizon would be a bug.

[mrBliss]

With IntersectMBO/ouroboros-consensus#681, the need for this will be significantly reduced.