Allow disconnections to be signaled by drivers

This allows for disconnections to be detected quickly
where possible.

It also makes sure other driver errors aren't thrown
in the event of a disconnection.

include/icsneo/device/device.h

@@ -49,6 +49,7 @@ public:
 	virtual bool close();
 	virtual bool isOnline() const { return online; }
 	virtual bool isOpen() const { return com->isOpen(); }
+	virtual bool isDisconnected() const { return com->isDisconnected(); }
 	virtual bool goOnline();
 	virtual bool goOffline();