Unified error type for sendTx

[j-mueller]

MonadBlockchain.sendTx returns an Either (ValidationError era) TxId. ValidationError looks like this:

data ValidationError era =
  ValidationErrorInMode !C.TxValidationErrorInCardanoMode
  | VExUnits !(ExUnitsError era)
  | PredicateFailures ![CollectError (C.ShelleyLedgerEra era)]
  | ApplyTxFailure !(ApplyTxError (C.ShelleyLedgerEra era))
  deriving anyclass Exception

The ValidationErrorInMode constructor is only used in the MonadBlockchain instance for MonadBlockchainCardanoNodeT. The other constructors are only used in the MockchainT instance. This means that

• Clients can discover which implementation is used (breaking the MonadBlockchain abstraction)
• Clients need to handle the same error in two different ways - once for the ValidationErrorInMode constructor, and then also for the other three constructors

To fix this we need to inspect the TxValidationErrorInCardanoMode that's returned from the node (MonadBlockchainCardanoNodeT) and try to extract the underlying reason.