Improve the "Try making some of these repositories available" message

With some hints for the user for what to do.

Took care to avoid changing the json output. It would have been ok to add
the new separated lists to it, in addition to the old list, but I didn't
do that because I didn't see much point.

Messages.hs

@@ -45,6 +45,7 @@ module Messages (
 	disableDebugOutput,
 	debugEnabled,
 	commandProgressDisabled,
+	jsonOutputEnabled,
 	outputMessage,
 	withMessageState,
 	prompt,
@@ -288,6 +289,12 @@ commandProgressDisabled = withMessageState $ \s -> return $
 		JSONOutput _ -> True
 		NormalOutput -> concurrentOutputEnabled s
 
+jsonOutputEnabled :: Annex Bool
+jsonOutputEnabled = withMessageState $ \s -> return $
+	case outputType s of
+		JSONOutput _ -> True
+		_ -> False
+
 {- Prevents any concurrent console access while running an action, so
  - that the action is the only thing using the console, and can eg prompt
  - the user.