Bug 15288: Error pages: Code duplication removal and better translatability

 Bug 15288: Error pages: Code duplication removal and better translatability

This patch changes the 400, 401, 402, 403, 404, 500 error pages to be handled
by only one template for staff and only one template for OPAC.

Additionally it removes a translatability issues due to sentence splitings by html tags.

To test:
- Apply patch
- Trigger 404 error for staff and for OPAC by calling a page that does not exist
- Try to trigger other error pages and/or carefully review code changes
  in the *.pl files
- Review koha-tmpl/opac-tmpl/bootstrap/en/modules/errors/errorpage.tt and
  koha-tmpl/intranet-tmpl/prog/en/modules/errorpage.tt

(Amended to fix a typo)
(Amended for comment #6)
(Amended to cover OPAC error pages as well)

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Works as advertised

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>