Use skebby test method for tests

There's not such thing as test accounts in Skebby, only option for
testing for free is to use a different method parameter.
Rework error handling

Instead of returning a dict, augment the requests' response with:
- skebby_error: boolean that indicates if an error was found or not
- skebby_message: error message
- skebby_content: a dict built from skebby response