If an exception occurs on Page Builder preview (e.g. by wrong template name used) context should be restored from previewed SiteAccess to Admin to display proper error template (from Admin).
The bug wasn't visible until eZ Design has been enabled.

Steps to reproduce:

1. Install ezplatform-ee-demo, run in prod ENV.
2. Break template name used for rendering e.g. embed blocks in app/config/default_layouts.yml
3. Create LP with that broken embed block, drop it on LP and submit.
4. See in Browser dev console a request that results in HTTP 500 response code, but contains an info about missing template "@main/errors/error.html.twig" instead of usual production ENV error message.