Capture replays on backend errors (MPAs, SSR)

[mydea]

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. <meta name="sentry-replay-error">).