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