To allow for passing moderation errors around without impacting the State, this commit also separates the previous State enum into two enums: State, and Error. The State enum now only contains generic states like disconnected or connected, and the Error enum describes the specific error happened.
citra_qt/multiplayer/{state, message} is changed accordingly.
Each window can still watch for state changes to update the ui or to
close the window as appropriate, but for any error announcements, they
all belong in Multiplayer state now.
