haserl-i18n: allow a translation language to skip entire pages

Pursuant to this commit:
https://github.com/ericpaulbishop/gargoyle/commit/87732a0f786cb092ef4c7be360282585e27c0dcf

This allows the die message when a page in the fallback_lang (typically English) is not present. As that would be a 'bad thing' I think alerting the developer to the issue would be invaluable.