db-analyser: fix `--trace-ledger` across eras

[amesgen]

When crossing an era boundary while using --trace-ledger, db-analyser crashes due to

ouroboros-consensus/ouroboros-consensus-cardano/src/tools/Cardano/Tools/DBAnalyser/Block/Cardano.hs

Line 97 in 4b668de

. fromJust

To see why, consider the signature of the offending function (that is used in emitTraces):

ouroboros-consensus/ouroboros-consensus-cardano/src/tools/Cardano/Tools/DBAnalyser/Block/Cardano.hs

Lines 87 to 93 in 4b668de

	-- | Lift a function polymorphic over all block types supporting `HasAnalysis`
	-- into a corresponding function over `CardanoBlock.`
	analyseWithLedgerState ::
	forall a.
	(forall blk. HasAnalysis blk => WithLedgerState blk -> a) ->
	WithLedgerState (CardanoBlock StandardCrypto) ->
	a

with

ouroboros-consensus/ouroboros-consensus-cardano/src/tools/Cardano/Tools/DBAnalyser/HasAnalysis.hs

Lines 24 to 28 in 4b668de

	data WithLedgerState blk = WithLedgerState
	{ wlsBlk :: blk
	, wlsStateBefore :: LedgerState blk
	, wlsStateAfter :: LedgerState blk
	}

where wlsState{Before,After} are ledger states of the same block/era, but when comparing the ledger states just before/after an era transition, that is not the case.

The minimum goal of this ticket is to make sure that we don't crash on era transitions.

• The straightforward approach would be to return Maybe a from analyseWithLedgerState and emit no traces on era transitions.
• Going further slightly, we might want to modify the emitTraces interface slightly such that one can also compare ledger states from different eras to e.g. still be able to mark the start of an epoch at the point of the era transition. Conceptually, that would involve providing functions WithLedgerState blk blk' -> a for all subsequent pairs of blocks in CardanoEras, cf. the existing InPairs combinator.