[BUGFIX] Inform about online media already exists

In case a user adds an online media asset,
which is already present in the target folder
a flash message now properly informs about
it instead of stating that the asset was
"added successfully".

Resolves: #101980
Releases: main, 12.4
Change-Id: Iae2bb05a22cc498a5f0bad1fa5e6723d0c2d99b1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81113


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>