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.)

Assistant/Restart.hs

@@ -95,7 +95,9 @@ newAssistantUrl repo = do
  - warp-tls listens to http, in order to show an error page, so this works.
  -}
 assistantListening :: URLString -> IO Bool
-assistantListening url = catchBoolIO $ exists url' =<< defUrlOptions
+assistantListening url = catchBoolIO $ do
+	uo <- defUrlOptions
+	(== Right True) <$> exists url' uo
   where
 	url' = case parseURI url of
 		Nothing -> url

Assistant/Upgrade.hs

@@ -40,9 +40,10 @@ import Utility.Metered
 import qualified Utility.Lsof as Lsof
 import qualified BuildInfo
 import qualified Utility.Url as Url
-import qualified Annex.Url as Url
+import qualified Annex.Url as Url hiding (download)
 import Utility.Tuple
 
+import Data.Either
 import qualified Data.Map as M
 
 {- Upgrade without interaction in the webapp. -}
@@ -323,8 +324,8 @@ downloadDistributionInfo = do
 	liftIO $ withTmpDir "git-annex.tmp" $ \tmpdir -> do
 		let infof = tmpdir </> "info"
 		let sigf = infof ++ ".sig"
-		ifM (Url.download nullMeterUpdate distributionInfoUrl infof uo
-			<&&> Url.download nullMeterUpdate distributionInfoSigUrl sigf uo
+		ifM (isRight <$> Url.download nullMeterUpdate distributionInfoUrl infof uo
+			<&&> (isRight <$> Url.download nullMeterUpdate distributionInfoSigUrl sigf uo)
 			<&&> verifyDistributionSig gpgcmd sigf)
 			( parseInfoFile <$> readFileStrict infof
 			, return Nothing

Assistant/WebApp/Configurators/IA.hs

@@ -192,7 +192,7 @@ escapeHeader = escapeURIString (\c -> isUnescapedInURI c && c /= ' ')
 getRepoInfo :: RemoteConfig -> Widget
 getRepoInfo c = do
 	uo <- liftAnnex Url.getUrlOptions
-	exists <- liftIO $ catchDefaultIO False $ Url.exists url uo
+	exists <- liftAnnex $ catchDefaultIO False $ Url.exists url uo
 	[whamlet|
 <a href="#{url}">
   Internet Archive item