separate <Layout> from <Error> in generate_app( )

[thebells1111]

_error.svelte will now be able to be styled independently of _layout.svelte to allow for unique error pages. This is in response to the comment by @intrikate in PR #948: _error.svelte should not inherit the common layout by default.

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Tests

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[Conduitry]

Thanks - but while we do want to have a way for pages to know whether they are an error page, I don't think we want to do it this way - which would be a breaking change in any case.

[ArekBartnik]

@Conduitry it would be a breaking change but easily fixable (move some components from _layout to _error)

I think the reasoning should be this - if _layout encapsulates correct pages, _error should do the same for error pages.
Now there's an inconsistent pattern.