[BUGFIX] Show ImageMagick command on failure

Install tool now shows an ImageMagick command which
failed.

Resolves: #83868
Releases: master
Change-Id: I4555d2ccdf68eea9180b8c7d2671b11e9b637709
Reviewed-on: https://review.typo3.org/55689


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>