decodeFileEither: propagate IOExceptions correctly

The interface we wrap expects only Yaml.ParseException to be thrown, and
to be wrapped inside Either. Conform to that.

src/Data/Yaml/Extra.hs

@@ -1,17 +1,21 @@
+{-# LANGUAGE ScopedTypeVariables #-}
 -- | Wrappers for Yaml functions to workaround
 -- https://github.com/commercialhaskell/stack/issues/2491.
 -- Import Data.Yaml.Extra in place of Data.Yaml to use this workaround.
 -- Beware these functions construct/deconstruct the entire file at once!
 module Data.Yaml.Extra (decodeFileEither, encodeFile, module Data.Yaml) where
 
+import           Control.Exception
 import           Data.Yaml hiding (decodeFileEither, encodeFile)
 import qualified Data.ByteString as B
 import           System.IO
 
 -- Note: we refrain from using 'B.readFile' and 'B.writeFile', as they open
 -- the file in binary mode rather than text mode.
 decodeFileEither :: FromJSON a => FilePath -> IO (Either ParseException a)
-decodeFileEither path = withFile path ReadMode $
+decodeFileEither path =
+  handle (\(e :: IOException) -> return . Left . OtherParseException . SomeException $ e) $
+  withFile path ReadMode $
   \hnd -> do
     fileContent <- B.hGetContents hnd
     return $ decodeEither' fileContent