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.)

Annex/Url.hs

@@ -1,24 +1,39 @@
 {- Url downloading, with git-annex user agent and configured http
  - headers, security restrictions, etc.
  -
- - Copyright 2013-2018 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2019 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
 module Annex.Url (
-	module U,
 	withUrlOptions,
 	getUrlOptions,
 	getUserAgent,
 	ipAddressesUnlimited,
+	checkBoth,
+	download,
+	exists,
+	getUrlInfo,
+	U.downloadQuiet,
+	U.URLString,
+	U.UrlOptions(..),
+	U.UrlInfo(..),
+	U.sinkResponseFile,
+	U.matchStatusCodeException,
+	U.downloadConduit,
+	U.downloadPartial,
+	U.parseURIRelaxed,
+	U.allowedScheme,
+	U.assumeUrlExists,
 ) where
 
 import Annex.Common
 import qualified Annex
-import Utility.Url as U
+import qualified Utility.Url as U
 import Utility.IPAddress
 import Utility.HttpManagerRestricted
+import Utility.Metered
 import qualified BuildInfo
 
 import Network.Socket
@@ -43,7 +58,7 @@ getUrlOptions = Annex.getState Annex.urloptions >>= \case
   where
 	mk = do
 		(urldownloader, manager) <- checkallowedaddr
-		mkUrlOptions
+		U.mkUrlOptions
 			<$> (Just <$> getUserAgent)
 			<*> headers
 			<*> pure urldownloader
@@ -108,3 +123,27 @@ ipAddressesUnlimited =
 
 withUrlOptions :: (U.UrlOptions -> Annex a) -> Annex a
 withUrlOptions a = a =<< getUrlOptions
+
+checkBoth :: U.URLString -> Maybe Integer -> U.UrlOptions -> Annex Bool
+checkBoth url expected_size uo =
+	liftIO (U.checkBoth url expected_size uo) >>= \case
+		Right r -> return r
+		Left err -> warning err >> return False
+
+download :: MeterUpdate -> U.URLString -> FilePath -> U.UrlOptions -> Annex Bool
+download meterupdate url file uo =
+	liftIO (U.download meterupdate url file uo) >>= \case
+		Right () -> return True
+		Left err -> warning err >> return False
+
+exists :: U.URLString -> U.UrlOptions -> Annex Bool
+exists url uo = liftIO (U.exists url uo) >>= \case
+	Right b -> return b
+	Left err -> warning err >> return False
+
+getUrlInfo :: U.URLString -> U.UrlOptions -> Annex U.UrlInfo
+getUrlInfo url uo = liftIO (U.getUrlInfo url uo) >>= \case
+	Right i -> return i
+	Left err -> do
+		warning err
+		return $ U.UrlInfo False Nothing Nothing