avoid warning when youtube-dl is not installed

If a user does not have it installed, don't warn on every imported item
about it.

Annex/YoutubeDl.hs

@@ -80,9 +80,12 @@ youtubeDlTo key url dest = do
 			return False
 		Right r -> return r
 
+youtubeDlSupported :: URLString -> Annex Bool
+youtubeDlSupported url = either (const False) id <$> youtubeDlCheck url
+
 -- Check if youtube-dl can still find media in an url.
-youtubeDlSupported :: URLString -> Annex (Either String Bool)
-youtubeDlSupported url = catchMsgIO $ do
+youtubeDlCheck :: URLString -> Annex (Either String Bool)
+youtubeDlCheck url = catchMsgIO $ do
 	opts <- map Param . annexYoutubeDlOptions <$> Annex.getGitConfig
 	let opts' = opts ++ [ Param url, Param "--simulate" ]
 	liftIO $ snd <$> processTranscript "youtube-dl" (toCommand opts') Nothing

Command/ImportFeed.hs

@@ -286,18 +286,13 @@ performDownload opts cache todownload = case location todownload of
 				return False
 			Right b -> return b
 
-	addmediafast linkurl mediaurl mediakey =
-		youtubeDlSupported linkurl >>= \case
-			Right True ->
-				rundownload linkurl ".m" $ \f -> do
+	addmediafast linkurl mediaurl mediakey = ifM (youtubeDlSupported linkurl)
+		( rundownload linkurl ".m" $ \f -> do
 			addWorkTree webUUID mediaurl f mediakey Nothing
 			return [mediakey]
-			Right False ->
-				performDownload opts cache todownload
+		, performDownload opts cache todownload
 			{ location = Enclosure linkurl }
-			Left msg -> do
-				warning msg
-				return False
+		)
 
 defaultTemplate :: String
 defaultTemplate = "${feedtitle}/${itemtitle}${extension}"

Remote/Web.hs

@@ -106,7 +106,7 @@ checkKey' key us = firsthit us (Right False) $ \u -> do
 	let (u', downloader) = getDownloader u
 	showChecking u'
 	case downloader of
-		YoutubeDownloader -> youtubeDlSupported u'
+		YoutubeDownloader -> youtubeDlCheck u'
 		_ -> do
 			Url.withUrlOptions $ catchMsgIO .
 				Url.checkBoth u' (keySize key)