[ticket/11911] Warn admins when there is no search index for the selected engine

PHPBB3-11911
Merge remote-tracking branch 'nickvergessen/ticket/11911' into develop

# By Joas Schilling
# Via Joas Schilling
* nickvergessen/ticket/11911:
  [ticket/11911] Warn admins when there is no search index for the selected engine