Return DisconnectReason in the DisconnectEvent for clients

[cBournhonesque]

Currently the user has no way of accessing the reason why the connection was not successful (or the reason they got disconnected). There are usually some error! logs printed, but the user might need to access the value to handle it in some ad-hoc way.

In this PR, we add:

• a DisconnectReason enum to the client's Disconnect event. It gets populated when we have an io-based error, netcode error or steam error
• the ClientConnection.state() now returns a ConnectionState instead of a NetworkingState

(- I also add a NetworkingState::None enum to avoid running the OnDisconnect schedule at the start of the app)

Addresses part of #410 (comment)