Check if template file exists

To prevent errors on frontend when a non-existing template file is used, I've added a check during template editing to check if file exists after update/save.
Same template file check as EditThemeTemplate.php
Added error message for NL and EN.
Changed error message ID to sync with locale.xml
Changed error message ID to sync with locale.xml
Merge pull request #898 from drsdre/patch-1

Check if template file exists