urgent fix : only open connection for images if not in local

should fix also #352

This is urgent because at the moment, each time a user gets to the deck overlay, the program opens a connection to amazon servers for the images. I think it overloads the server and causes a lot of issues.

Now only images not already downloaded will trigger ```Channels.newChannel(new URL(card.url).openStream)```

also better handling of errors in this part of the code.