make the danger in `epochInfo` more apparent.

We should document and make clearer the circumstances when the methods associated with `epochInfo` return `Left`.

In particular, we often do the following to get the first slot of the next epoch:
```
ei <- liftSTS $ asks epochInfo
EpochNo e <- liftSTS $ epochInfoEpoch ei s
let newEpochNo = EpochNo (e + 1)
firstSlotNextEpoch <- liftSTS $ epochInfoFirst ei newEpochNo
```

This construction works, but would not, for example, if we let `newEpochNo = EpochNo (e + 2)`. This construction could perhaps be a good standalone function.