Make original pageContext accessible to error page

[brillout]

Description

Use case: i18n, see #1110.

In principle, it should be possible to make the original pageContext (of the page that failed to render) available to the error page's pageContext, e.g. at pageContext.pageContextOriginal.

Ideally, it should be automatically merged to the error pageContext so that it "just works", while preserving a pristine pageContext at pageContext.pageContextError.

[fvanwijk]

Alternatively instead of merging, you could make the original pageContext available on the error's pageContext.originalPageContext.

[brillout]

Actually, in general, I'm thinking the best is to do both: merge everything into pageContext while providing a pageContext.from. See #1268.