make --json-error-messages capture url download errors

Convert Utility.Url to return Either String so the error message can be
displated in the annex monad and so captured.

(When curl is used, its errors are still not caught.)

Remote/Web.hs

@@ -116,9 +116,8 @@ checkKey' key us = firsthit us (Right False) $ \u -> do
 	showChecking u'
 	case downloader of
 		YoutubeDownloader -> youtubeDlCheck u'
-		_ -> do
-			Url.withUrlOptions $ liftIO . catchMsgIO .
-				Url.checkBoth u' (keySize key)
+		_ -> catchMsgIO $
+			Url.withUrlOptions $ Url.checkBoth u' (keySize key)
   where
 	firsthit [] miss _ = return miss
 	firsthit (u:rest) _ a = do