Refactor error pages, minus the merge conflicts

[afilina]

This is an attempt to push a stalled PR through. https://github.com/php/doc-en/pull/320

Since there are years of changes to files that were deleted in the initial PR, this one "accepts theirs" for any conflicts. Once the bulk is merged, we'll do a more granular pass on the conflicts, especially when it comes to replicating any info that was added over the years that needs to exist somewhere before we mass-delete pages.

I'd keep the other PR until we're done incorporating the changes. This approach also keeps credit where it's due for commits.