make retrieveKeyFile and retrieveKeyFileCheap throw exceptions

Converted retrieveKeyFileCheap to a Maybe, to avoid needing to throw a
exception when a remote doesn't support it.

Remote/Ddar.hs

@@ -71,8 +71,8 @@ gen r u rc gc rs = do
 		, cost = cst
 		, name = Git.repoDescribe r
 		, storeKey = storeKeyDummy
-		, retrieveKeyFile = retreiveKeyFileDummy
-		, retrieveKeyFileCheap = retrieveCheap
+		, retrieveKeyFile = retrieveKeyFileDummy
+		, retrieveKeyFileCheap = Nothing
 		-- ddar communicates over ssh, not subject to http redirect
 		-- type attacks
 		, retrievalSecurityPolicy = RetrievalAllKeysSecure
@@ -162,9 +162,6 @@ retrieve ddarrepo = byteRetriever $ \k sink -> do
 	liftIO (hClose h >> forceSuccessProcess p pid)
 		`after` (sink =<< liftIO (L.hGetContents h))
 
-retrieveCheap :: Key -> AssociatedFile -> FilePath -> Annex Bool
-retrieveCheap _ _ _ = return False
-
 remove :: DdarRepo -> Remover
 remove ddarrepo key = do
 	(cmd, params) <- ddarRemoteCall NoConsumeStdin ddarrepo 'd'