display error when an invalid url is downloaded

download is documented as displaying an error when download fails, but
it didn't when the url was not valid at all. That leads to confusing
behavior.

Also, display the url with --debug

Utility/Url.hs

@@ -311,7 +311,10 @@ download meterupdate url file uo =
 				(DownloadWithCurl _, _)
 					| isfileurl u -> downloadfile u
 					| otherwise -> downloadcurl
-		Nothing -> return False
+		Nothing -> do
+			liftIO $ debugM "url" url
+			hPutStrLn stderr "download failed: invalid url"
+			return False
 	
 	isfileurl u = uriScheme u == "file:"