Better error message for a non-deserializable transaction

[michalrus]

Reproduction

@vladimirvolek noticed (GitHub Actions run), that if you send a non-deserializable transaction to cardano-node, you'll get a cryptic ChannelError(AgentDequeue) error, e.g.:

❯ curl -fsSL http://0:3000/tx/submit -X POST -H "Content-Type: application/cbor" -d "aaaa" | jq .
{
  "status_code": 400,
  "error": "Bad Request",
  "message": "Error during transaction submission: ChannelError(AgentDequeue)",
  "details": null
}

In cardano-node’s log, you can see a more proper error, (DeserialiseFailure 9 "expected list len or indef"):

[lenovo-x:cardano.node.LocalInboundGovernor:Info:77] [2025-01-22 12:08:23.41 UTC] TrResponderErrored (ConnectionId {localAddress = LocalAddress "/home/mw/.local/share/blockfrost-platform/preview/node.socket", remoteAddress = LocalAddress "/home/mw/.local/share/blockfrost-platform/preview/node.socket@35"}) (MiniProtocolNum 6) DecoderFailure (LocalTxSubmission (GenTx (HardForkBlock (': * ByronBlock (': * (ShelleyBlock (TPraos StandardCrypto) (ShelleyEra StandardCrypto)) (': * (ShelleyBlock (TPraos StandardCrypto) (AllegraEra StandardCrypto)) (': * (ShelleyBlock (TPraos StandardCrypto) (MaryEra StandardCrypto)) (': * (ShelleyBlock (TPraos StandardCrypto) (AlonzoEra StandardCrypto)) (': * (ShelleyBlock (Praos StandardCrypto) (BabbageEra StandardCrypto)) (': * (ShelleyBlock (Praos StandardCrypto) (ConwayEra StandardCrypto)) ('[] *)))))))))) (HardForkApplyTxErr (': * ByronBlock (': * (ShelleyBlock (TPraos StandardCrypto) (ShelleyEra StandardCrypto)) (': * (ShelleyBlock (TPraos StandardCrypto) (AllegraEra StandardCrypto)) (': * (ShelleyBlock (TPraos StandardCrypto) (MaryEra StandardCrypto)) (': * (ShelleyBlock (TPraos StandardCrypto) (AlonzoEra StandardCrypto)) (': * (ShelleyBlock (Praos StandardCrypto) (BabbageEra StandardCrypto)) (': * (ShelleyBlock (Praos StandardCrypto) (ConwayEra StandardCrypto)) ('[] *)))))))))) ClientAgency TokIdle) (DeserialiseFailure 9 "expected list len or indef")

So it seems that a non-deserializable transaction somehow breaks the Ouroboros flow in Pallas?

Desired outcome

We should have a better error message, if getting to (DeserialiseFailure 9 "expected list len or indef") is not possible easily, then maybe just something generic like "Invalid transation"?