[BUGFIX] Render image processing results in install tool

Some checks might return a message why images can't be generated.
Those checks didn't return success which prevents processing by
JavaScript, leading to endless "Loading" state within UI.
All responses now properly return success = true in order to be
processed by JavaScript.

Resolves: #100893
Releases: main, 12.4
Change-Id: I720893a0b01a26a7b77bdcb16c02295eed3ae595
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79680


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>