Bug 15288: (followup) Better wording for OPAC error page

 Bug 15288: (followup) Better wording for OPAC error page

This patch changes the wording of the OPAC error page (See Bug 14573).

Additionally, it displays the link to the login page only if the system
preference 'opacuserlogin' is set.

To test:
- Apply patch
- Review koha-tmpl/opac-tmpl/bootstrap/en/modules/errors/errorpage.tt

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
No errors

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Amended: Replaced following reasons by 'the following reasons'. Reads better.

Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>