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/P2P.hs

@@ -56,7 +56,7 @@ chainGen addr r u rc gc rs = do
 		, name = Git.repoDescribe r
 		, storeKey = store (const protorunner)
 		, retrieveKeyFile = retrieve (const protorunner)
-		, retrieveKeyFileCheap = \_ _ _ -> return False
+		, retrieveKeyFileCheap = Nothing
 		, retrievalSecurityPolicy = RetrievalAllKeysSecure
 		, removeKey = remove protorunner
 		, lockContent = Just $ lock withconn runProtoConn u