Bug 13948: Prevent explosion when Template::Plugin::Stash not installed

Bug 13948: Prevent explosion when Template::Plugin::Stash not installed

TEST PLAN
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1) Install first two patches
2) do not install, or uninstall Template::Plugin::Stash
3) Upgrade to make sure system preference is added.
4) Set the system preference to turn it on for Staff and OPAC
5) Refresh staff -- kaboom
6) Load OPAC -- kaboom
7) Apply this patch
8) Reload staff and OPAC
   -- nice HTML comment about what is wrong.
9) run koha qa test tools.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Neat, runs well. Tested with/without sysprefs and Template::Plugin::Stash
No koha-qa errors

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>