Add error handling for mtp and gpod device

src/device/connecteddevice.h

@@ -50,6 +50,7 @@ class ConnectedDevice : public QObject,
   ~ConnectedDevice();
 
   virtual bool Init() = 0;
+  virtual void ConnectAsync();
   // For some devices (e.g. CD devices) we don't have callbacks to be notified when something change:
   // we can call this method to refresh device's state
   virtual void Refresh() {}
@@ -71,6 +72,7 @@ class ConnectedDevice : public QObject,
  signals:
   void TaskStarted(int id);
   void SongCountUpdated(int count);
+  void ConnectFinished(const QString& id, bool success);
 
  protected:
   void InitBackendDirectory(const QString &mount_point, bool first_time, bool rewrite_path = true);