Bug 25346: Supress warning message when install dir exists

Bug 25346: Supress warning message when install dir exists

To see it enable verbose mode

To test:
1) Install lang with install dir
   (cd misc/translator; ./tranlste install de-DE)
   check message: "de-DE installer dir /...mysql/de-DE already exists."
2) Apply patch
3) Repeat 1, now message is gone
4) Repeat 1 in verbose mode (./translate install de-DE -v)
   check the message now appears at the bottom.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>

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