[BUGFIX] Allow silent upgrade service to migrate old displayErrors setting

The bootstrap throws an exception when the setting SYS/displayErrors
is not set to "-1", "0" or "1". In the Install Tool TYPO3 should not crash.
Instead allow to silently migrate the value to -1

Resolves: #82782
Resolves: #83787
Releases: master,8.7
Change-Id: Ie0180b34a551a81667bb4ac32c6ab2699d9b3d4f
Reviewed-on: https://review.typo3.org/55619


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>