Bug 17989: Centralize bad template check

Bug 17989: Centralize bad template check

The bad template check in get_template_and_user can be removed, since
this check has been added in gettemplate on bug 18010.

The check moves here to a new sub in C4::Templates. And will be extended
in a follow-up.

Removed unused variable $path in gettemplatefile.

Test plan:
[1] Run t/db_dependent/Auth.t (get_template_and_user bad calls).
[2] Run t/db_dependent/Templates.t (bad call to gettemplate).

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>