robustify error handling

[Rich-Harris]

Currently, if you introduce an error into a component that causes rendering to fail...

<p>{this_variable_does_not_exist}</p>

...fixing the component won't cause the page to recover, because Svelte itself is now in a broken state. (We plan to make Svelte more resilient to this sort of thing, but it's a ways off.) Adding hot.optimistic: false to the vite-plugin-svelte options causes the page to reload on HMR updates after a rendering error is encountered.

That solves half the problem. If the page reloads and it's still broken, the server will render an error page. Since the error page generally won't use the component where the error was, subsequent HMR updates will be ignored, so fixing the component won't cause the error page to go away.

I think a reasonable way to fix that is to always reload the page when an HMR update occurs, if you're on an error page.

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