[TASK] Streamline drag uploader failure message

The Folder Tree shows notifications through
FlashMessages when using the AJAX endpoint "file_process",
while the drag uploader still used the very old
"terr" XML Response (which I added in TYPO3 v4.1).

Now the file_process returns the error messages
from the flash message queue, ready to be printed
out.

The "includeMessages" is thus not needed anymore,
as message results are always sent.

Resolves: #97177
Releases: main
Change-Id: I0ded504fcfd4847e01f44440fab5230899881285
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73906


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>