Capture replays on backend errors (MPAs, SSR)

Currently for MPAs error-based replay sampling will not necessarily work when the error is not thrown in JS.

We _can_ do this manually today (users can call `replay.flush()`), but maybe we can add some more/better logic there.

For example, when a page is rendered with a non-200 status code, we could consider this as an error and flush immediately.

And/or we could add another meta tag to the rendered HTML that tells replay that an error happened server side (e.g. ``).