[BUGFIX] Ajax handler TYPO3_tcefile::process is broken

Several changes in core broke the ajax upload handler response.
There is no error response and it tries to json_encode complete
File objects.

This changes reverts some of the "cleanup" changes fixes the
response and makes sure only the error message from current
request are used.

Resolves: #55371
Releases: 6.2, 6.1
Change-Id: Ia09f5ffd5e002b1ba50d4fafa747340462d15148
Reviewed-on: https://review.typo3.org/27080
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter