improve exception handling

Push it down from needing to be done in every Storer,
to being checked once inside ChunkedEncryptable.

Also, catch exceptions from PrepareStorer and PrepareRetriever,
just in case..

Remote/Helper/Chunked.hs

@@ -70,12 +70,16 @@ numChunks = pred . fromJust . keyChunkNum . fst . nextChunkKeyStream
  - the storer action, along with a corresponding chunk key and a
  - progress meter update callback.
  -
- - Note that the storer action is responsible for catching any
- - exceptions it may encounter.
- -
  - This action may be called on a chunked key. It will simply store it.
  -}
-storeChunks :: UUID -> ChunkConfig -> Key -> FilePath -> MeterUpdate -> (Key -> L.ByteString -> MeterUpdate -> IO Bool) -> Annex Bool
+storeChunks
+	:: UUID
+	-> ChunkConfig
+	-> Key
+	-> FilePath
+	-> MeterUpdate
+	-> (Key -> L.ByteString -> MeterUpdate -> IO Bool)
+	-> Annex Bool
 storeChunks u chunkconfig k f p storer = metered (Just p) k $ \meterupdate ->
 	either (\e -> warning (show e) >> return False) (go meterupdate)
 		=<< (liftIO $ tryIO $ L.readFile f)
@@ -188,7 +192,7 @@ retrieveChunks
 	-> Annex Bool
 retrieveChunks retriever u chunkconfig encryptor basek basep sink = do
 	ls <- chunkKeys u chunkconfig basek
-	liftIO $ flip catchNonAsync giveup (firstavail ls)
+	liftIO $ firstavail ls `catchNonAsync` giveup
   where
 	giveup e = do
 		warningIO (show e)