Make XPointer evaluation failure a hard error.

[alfsb]

This change makes XPointer evaluation failure a hard error on en/, while keeping it as a warning on translations

Also, improves --disable-xpointer-reporting in another place.

[alfsb]

This and https://github.com/php/doc-base/pull/98 supersedes:

• https://github.com/php/doc-base/pull/80
• https://github.com/php/doc-base/pull/85
• https://github.com/php/doc-en/pull/2015