Fix decoding of Hydra keys and ignore problematic head init

We were not correctly decoding Hydra verification keys (but crashing via
error) because of a missing data constructor.

This also makes the observeInitTx sort out heads that would have such
problematic party datums.

An example transaction with problematic data is:
b860a236a7e77577628bf705286d449188831cf90d714934f1cc06369c6e3953 on preprod

hydra-tx/src/Hydra/Tx/Crypto.hs

@@ -56,6 +56,7 @@ import Hydra.Cardano.Api (
   Key (..),
   SerialiseAsCBOR,
   SerialiseAsRawBytes (..),
+  SerialiseAsRawBytesError (..),
   serialiseToRawBytesHexText,
  )
 import Hydra.Contract.HeadState qualified as OnChain
@@ -83,7 +84,10 @@ instance SerialiseAsRawBytes (Hash HydraKey) where
   serialiseToRawBytes (HydraKeyHash vkh) = hashToBytes vkh
 
   deserialiseFromRawBytes (AsHash AsHydraKey) bs =
-    maybe (error "TODO: SerialiseAsRawBytesError, but constructor not exported") (Right . HydraKeyHash) (hashFromBytes bs)
+    maybe
+      (Left $ SerialiseAsRawBytesError "invalid length when deserializing Hash HydraKey")
+      (Right . HydraKeyHash)
+      (hashFromBytes bs)
 
 instance Key HydraKey where
   -- Hydra verification key, which can be used to 'verify' signed messages.
@@ -140,7 +144,10 @@ instance SerialiseAsRawBytes (VerificationKey HydraKey) where
     rawSerialiseVerKeyDSIGN vk
 
   deserialiseFromRawBytes (AsVerificationKey AsHydraKey) bs =
-    maybe (error "TODO: SerialiseAsRawBytesError, but constructor not exported") (Right . HydraVerificationKey) (rawDeserialiseVerKeyDSIGN bs)
+    maybe
+      (Left $ SerialiseAsRawBytesError "invalid length when deserializing VerificationKey HydraKey")
+      (Right . HydraVerificationKey)
+      (rawDeserialiseVerKeyDSIGN bs)
 
 instance ToJSON (VerificationKey HydraKey) where
   toJSON = toJSON . serialiseToRawBytesHexText

hydra-tx/src/Hydra/Tx/Observe.hs

@@ -53,6 +53,10 @@ data HeadObservation
 -- | Observe any Hydra head transaction.
 observeHeadTx :: NetworkId -> UTxO -> Tx -> HeadObservation
 observeHeadTx networkId utxo tx =
+  -- XXX: This is throwing away valuable information! We should be collecting
+  -- all "not an XX" reasons here in case we fall through and want that
+  -- diagnostic information in the call site of this function. Collecting errors
+  -- could be done with 'validation' or a similar package.
   fromMaybe NoHeadTx $
     either (const Nothing) (Just . Init) (observeInitTx tx)
       <|> Abort <$> observeAbortTx utxo tx