* Enhance system font stack on _error.js
`-apple-system` = SF on Safari (macOS/iOS)
`BlinkMacSystemFont` = SF on Chrome (macOS)
`Roboto` for Android
`Segoe UI` for Windows
`Fira Sans` for Firefox OS
`Avenir` for pre-SF macOS
`Helvetica Neue, Lucida Grande` for older macOS
* Enhance system font stacks on _error-debug.js
Updates fonts to use better quality system fonts where commonly available
The CSS rule on the error page doesn't work and the text on the page isn't rendered in SF UI Text. If we want to use SF on the error page, then, at least in Safari, we can use `-apply-system` as the font family.
I also added a fallback to sans-serif, just to be explicit.
