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/WebDAV.hs

@@ -84,8 +84,8 @@ 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
@@ -162,9 +162,6 @@ finalizeStore dav tmp dest = do
 	maybe noop (void . mkColRecursive) (locationParent dest)
 	moveDAV (baseURL dav) tmp dest
 
-retrieveCheap :: Key -> AssociatedFile -> FilePath -> Annex Bool
-retrieveCheap _ _ _ = return False
-
 retrieve :: DavHandleVar -> ChunkConfig -> Retriever
 retrieve hv cc = fileRetriever $ \d k p ->
 	withDavHandle hv $ \dav -> case cc of