[BUGFIX] Message when trying to create existing folder

Show a flash message when trying to create a folder with a name that
already exists.

Releases: master
Resolves: #14509
Change-Id: Ia293ea61a3a892103c9b82affe5f527f1c05b39e
Reviewed-on: http://review.typo3.org/36733


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>