Avoid losing in-progress work

[PartyPlanner64]

PP64 tries to save the board list into localstorage when the window closes. In most browsers, this is a limited resource, and if the board data grows to large, the write fails and work is lost.

Ideally there would at least be one of those prompts "you have unsaved work, etc." that would appear. I couldn't get it to work initially.

The workaround is to export anything important before closing the window.

[PartyPlanner64]

I at least got the prompt to work when localstorage saving fails, so might as well close this. Commit 2f491ea.