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.)

Command/ImportFeed.hs

@@ -146,13 +146,12 @@ findDownloads u f = catMaybes $ map mk (feedItems f)
 downloadFeed :: URLString -> Annex (Maybe String)
 downloadFeed url
 	| Url.parseURIRelaxed url == Nothing = giveup "invalid feed url"
-	| otherwise = Url.withUrlOptions $ \uo ->
-		liftIO $ withTmpFile "feed" $ \f h -> do
-			hClose h
-			ifM (Url.download nullMeterUpdate url f uo)
-				( Just <$> readFileStrict f
-				, return Nothing
-				)
+	| otherwise = withTmpFile "feed" $ \f h -> do
+		liftIO $ hClose h
+		ifM (Url.withUrlOptions $ Url.download nullMeterUpdate url f)
+			( Just <$> liftIO (readFileStrict f)
+			, return Nothing
+			)
 
 performDownload :: ImportFeedOptions -> Cache -> ToDownload -> Annex Bool
 performDownload opts cache todownload = case location todownload of
@@ -164,7 +163,7 @@ performDownload opts cache todownload = case location todownload of
 					urlinfo <- if relaxedOption (downloadOptions opts)
 						then pure Url.assumeUrlExists
 						else Url.withUrlOptions $
-							liftIO . Url.getUrlInfo url
+							Url.getUrlInfo url
 					let dlopts = (downloadOptions opts)
 						-- force using the filename
 						-- chosen here