Improve error handling when a note fails to render

[cloverich]

When a note fails to render, it displays a stack trace in the UI. This is kind of useful, but also leaves the user (me) stuck on that document page, unable to navigate away while also unable to really do anything about it. Instead do this:

1. Display the source document that caused the error (which is a file user can view on disk if needed)
2. Make it look a little nicer (easier to copy paste stack)
3. Put a back button on the error page
4. Put instructions on how to report the error (Github I guess, include stack + note content)