general: legacy directory pre-creation

* Ensures legacy directories are pre-created
  upon legacy extension setup_app() call.
  (closes #1789)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Yes, I thought about it, but it in this context, we are not really interested into the whole backtrace. Clearly there are going to be issues related to disk space, or permissions. But nothing that the backtrace can help the casual admin with... So I think it's better not to bloat his system logs...