Implement API 1.2 new endpoint Session#Check

[InfectedBytes]

Superseed loicteixeira#25

Waiting for gamejolt/doc-game-api#23 to be fixed

So the problem is basically that GameJolt misuses the success argument.
The success argument determines whether the request has succeeded or failed, but for the Session.Check call, they also use this value to say that there is no open session. So if it is false, it does not necessarily mean that the request has failed, it might also just mean that there is no open session.

[InfectedBytes]

It would be good if GameJolt fixes this, but for now I just go for a workaround:
If a request has actually failed, success is false and there is an additional message argument describing the problem. So if success is false, but there is no message argument, we can assume that the call has succeeded and we just don't have an open session.