added some debug informations if parsed string was not as expected

[phgachoud]

I was struggling some time to get errors informations with your example, had to debug your code to understand what you were expecting as string so would have been much easier for me to get it like that ;-)

[jocelyn]

Is there really a json error in this example? I guess no, so no need for this error message, however I keep this merge as it can be useful as example, especially if one changes the JSON content for testing. Thanks