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/Directory.hs

@@ -117,7 +117,7 @@ store d chunkconfig k b p = do
 	void $ tryIO $ createDirectoryIfMissing True tmpdir
 	case chunkconfig of
 		LegacyChunks chunksize -> Legacy.store chunksize finalizer k b p tmpdir destdir
-		_ -> flip catchNonAsync (\e -> warningIO (show e) >> return False) $ do
+		_ -> do
 			let tmpf = tmpdir </> keyFile k
 			meteredWriteFile p tmpf b
 			finalizer tmpdir destdir