Make the MockChain submission behave the same as the Direct one

By using the FailedToPostTx exception, hydra nodes will handle this
event and not crash.

Indeed in some situations it is actually expected that the submission
fails (e.g. multiple collect transactions)

Furthermore, this highlights that the MockChain behaves quite different
than the real cardano chain interfaced through a cardano-node (see FIXME)

hydra-node/test/Hydra/Model/MockChain.hs

@@ -26,12 +26,11 @@ import Data.Sequence (Seq (Empty, (:|>)))
 import Data.Sequence qualified as Seq
 import Data.Time (secondsToNominalDiffTime)
 import Data.Time.Clock.POSIX (posixSecondsToUTCTime)
-import GHC.IO.Exception (userError)
 import Hydra.BehaviorSpec (
   SimulatedChainNetwork (..),
  )
 import Hydra.Cardano.Api.Pretty (renderTxWithUTxO)
-import Hydra.Chain (Chain (..), initHistory)
+import Hydra.Chain (Chain (..), PostTxError (..), initHistory)
 import Hydra.Chain.Direct.Fixture (testNetworkId)
 import Hydra.Chain.Direct.Handlers (
   ChainSyncHandler (..),
@@ -147,17 +146,17 @@ mockChainAndNetwork tr seedKeys commits = do
             -- TODO: dry with block tx validation
             case evaluateTx tx utxo of
               Left err ->
-                throwSTM . userError . toString $
+                throwSTM . FailedToPostTx @Tx $
                   unlines
-                    [ "Invalid tx submitted"
+                    [ "MockChain: Invalid tx submitted"
                     , "Tx: " <> toText (renderTxWithUTxO utxo tx)
                     , "Error: " <> show err
                     ]
               Right report
                 | any isLeft report ->
-                    throwSTM . userError . toString $
+                    throwSTM . FailedToPostTx @Tx $
                       unlines
-                        [ "Invalid tx submitted"
+                        [ "MockChain: Invalid tx submitted"
                         , "Tx: " <> toText (renderTxWithUTxO utxo tx)
                         , "Error: " <> show (lefts . toList $ report)
                         ]
@@ -239,6 +238,10 @@ mockChainAndNetwork tr seedKeys commits = do
       Nothing ->
         pure ()
 
+  -- FIXME: This should actually work more like a chain fork / switch to longer
+  -- chain. That is, the ledger switches to the longer chain state right away
+  -- and we issue rollback and forwards to synchronize clients. However,
+  -- submission will already validate against the new ledger state.
   rollbackAndForward nodes chain numberOfBlocks = do
     doRollBackward nodes chain numberOfBlocks
     replicateM_ (fromIntegral numberOfBlocks) $