Use a typed JSON result for the `/api/pull-secret` API endpoints

[gbraad]

Instead of an empty or bare message, we should return a JSON message from the API: crc-org/crc#2648

For this to happen we need to change the following code:

https://github.com/code-ready/tray-windows/blob/482f4dafec482add25cbdbd92567521426807745/Communication/DaemonCommander.cs#L90

... which can be simplified to a basic response without the specific exception handling.