Bug fixes for 2.3.1

Fix a few bugs in 2.3:

* Occasional NPE crash when closing preferences
* Fix adding images from Google in multimedia editor
* Force media rescan when IO exception occurs during media sync

@dae, @hssm 
2.3 currently fails to complete media sync if a user has manually a deleted a media file... It doesn't crash, but an [unhandled exception dialog](https://lh5.googleusercontent.com/-oDS5_ebfpaM/VFFBHrQN8CI/AAAAAAAAEho/d_NCcr3Xf4o/s320/Screenshot_2014-10-29-19-16-05.png) is shown, and the user isn't given any clues on how to solve the problem.

In 8cbfa15 I just set `dirmod` so that next media sync the database will be rebuilt, which will automatically resolve the problem. I know we are forced to do things a bit differently from Anki Desktop, so could you guys please confirm if this is the right thing to do?