Add status and error to page store

[tanhauhau]

related to #598

not sure how to add test for this

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Latest commit: ab378c9

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[benmccann]

The CI is failing on this line. You'll need to make sure pnpm check passes

[Rich-Harris]

Since we've got rid of the page object in load functions in #3126, and stores.page now contains url and params directly, @Conduitry made the excellent observation that it should also include error and status rather than us having separate stores. This also conveniently solves any potential issues around stores getting out of sync (e.g. you're explicitly subscribing to both error and status and don't want to experience tearing)