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/Hook.hs

@@ -59,8 +59,8 @@ gen r u rc gc rs = do
 			, cost = cst
 			, name = Git.repoDescribe r
 			, storeKey = storeKeyDummy
-			, retrieveKeyFile = retreiveKeyFileDummy
-			, retrieveKeyFileCheap = retrieveCheap hooktype
+			, retrieveKeyFile = retrieveKeyFileDummy
+			, retrieveKeyFileCheap = Nothing
 			-- A hook could use http and be vulnerable to
 			-- redirect to file:// attacks, etc.
 			, retrievalSecurityPolicy = mkRetrievalVerifiableKeysSecure gc
@@ -162,9 +162,6 @@ retrieve h = fileRetriever $ \d k _p ->
 	unlessM (runHook' h "retrieve" k (Just d) $ return True) $
 		giveup "failed to retrieve content"
 
-retrieveCheap :: HookName -> Key -> AssociatedFile -> FilePath -> Annex Bool
-retrieveCheap _ _ _ _ = return False
-
 remove :: HookName -> Remover
 remove h k = runHook' h "remove" k Nothing $ return True