Allow exporting without card backs

[maeligg]

Hello,

I typically don't print card backs for custom cards, since they go in opaque card sleeves anyways (and for TTS I use a single image for all cards so I don't really need this tool). I can't imagine that I'm the only one, so it might be interesting to support this use case ? At the very least, it would be nice to have a relevant error message when trying to export cards when the back template isn't defined. I couldn't figure out initially that this was why the export was failing.