Handle storage failure

[LeopoldTal]

The doc for storage.sync says

You can store up to 100KB of data using this API. If you try to store more than this, the call will fail with an error message.

But I haven't managed to make it fail. It just gets slow.

I also don't know what failures other than "out of space" can happen.

Need to think about how to handle a failure. Silently continuing to update the list, but not saving it, sounds Bad. Plain refusing to work isn't much better.