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/S3.hs

@@ -193,8 +193,8 @@ gen r u rc gc rs = do
 			, cost = cst
 			, name = Git.repoDescribe r
 			, storeKey = storeKeyDummy
-			, retrieveKeyFile = retreiveKeyFileDummy
-			, retrieveKeyFileCheap = retrieveCheap
+			, retrieveKeyFile = retrieveKeyFileDummy
+			, retrieveKeyFileCheap = Nothing
 			-- HttpManagerRestricted is used here, so this is
 			-- secure.
 			, retrievalSecurityPolicy = RetrievalAllKeysSecure
@@ -418,9 +418,6 @@ retrieveHelper' h f p req = liftIO $ runResourceT $ do
 	S3.GetObjectResponse { S3.gorResponse = rsp } <- sendS3Handle h req
 	Url.sinkResponseFile p zeroBytesProcessed f WriteMode rsp
 
-retrieveCheap :: Key -> AssociatedFile -> FilePath -> Annex Bool
-retrieveCheap _ _ _ = return False
-
 remove :: S3HandleVar -> Remote -> S3Info -> Remover
 remove hv r info k = withS3HandleOrFail (uuid r) hv $ \h -> liftIO $ runResourceT $ do
 	res <- tryNonAsync $ sendS3Handle h $