Provide users with information in the case of a crash (panic)

[Keavon]

If the Graphite Editor backend panics, the user has to open up the JS console to learn what happened. It will otherwise just look like most things have frozen. At a minimum, we should show a dialog when the backend panics, with a button to reload the page.

If can can display information about the crash, like logs that can be reported and a link to file an issue here, that would be helpful.

If it's possible, it would be even better if we can provide users a way to download their documents so their hard work isn't lost. Or automatically reload the Graphite Editor wasm backend and reopen the saved documents automatically.