conway governance hash: Correct/introduce error handling

cardano-cli/src/Cardano/CLI/EraBased/Run/Governance/Hash.hs

@@ -48,16 +48,18 @@ runGovernanceHashCmd Cmd.GovernanceHashCmdArgs { toHash } =
   -- This can be done in a separate PR
   case toHash of
     Cmd.GovernanceHashSourceBinaryFile fp -> do
-      bytes <- liftIO $ BS.readFile $ unFile fp
+      let path = unFile fp
+      bytes <- handleIOExceptT (GovernanceHashReadFileError path) $ BS.readFile path
       let hash = Ledger.hashAnchorData $ Ledger.AnchorData bytes
       printHash hash
     Cmd.GovernanceHashSourceTextFile fp -> do
-      text <- liftIO $ Text.readFile $ unFile fp
+      let path = unFile fp
+      text <- handleIOExceptT (GovernanceHashReadFileError path) $ Text.readFile path
       let hash = Ledger.hashAnchorData $ Ledger.AnchorData $ Text.encodeUtf8 text
       printHash hash
     Cmd.GovernanceHashSourceText text -> do
       let hash = Ledger.hashAnchorData $ Ledger.AnchorData $ Text.encodeUtf8 text
       printHash hash
   where
-    printHash :: Ledger.SafeHash StandardCrypto i -> ExceptT GovernanceHashError IO ()
+    printHash :: Ledger.SafeHash StandardCrypto i -> ExceptT a IO ()
     printHash = liftIO . putStr . Text.unpack . hashToTextAsHex . extractHash

cardano-cli/src/Cardano/CLI/Types/Errors/GovernanceHashError.hs

@@ -1,15 +1,17 @@
+{-# LANGUAGE LambdaCase #-}
 module Cardano.CLI.Types.Errors.GovernanceHashError
   ( GovernanceHashError(..)
   ) where
 
 import           Cardano.Api
 
-import           Cardano.Prelude (UnicodeException)
+import           Cardano.Prelude (Exception (displayException), IOException)
 
 data GovernanceHashError
-  = GovernanceHashReadFileError (FileError InputDecodeError)
-  | GovernanceHashUnicodeError UnicodeException
+  = GovernanceHashReadFileError FilePath IOException
   deriving Show
 
 instance Error GovernanceHashError where
-  displayError = undefined
+  displayError = \case
+    GovernanceHashReadFileError filepath exc ->
+      "Cannot read " <> filepath <> ": " <> displayException exc