Bug 31211: Check slips and notices for valid Template Toolkit and report errors

Bug 31211: Check slips and notices for valid Template Toolkit and report errors

It would be useful for Koha to run a basic TT validation test and report back errors.
That way if somebody misses an END tag or something of that nature, it will be easy to find and fix.

Test Plan:
1) Apply this patch
2) Edit a notice, change the content to simply "[% END %]"
   or some other invalid Template Toolkit
3) Choose "Save and continue editing"
4) Note the new "Template Toolkit error" field that displays the error
   message

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>