[TASK] Install tool: Improve ajax error handling

Show an error message if the server answer ajax calls
with http status code 200. This could happen if a proxy
responses to the http request or a network failures
occures.

Releases: master
Resolves: #82388

Change-Id: Ie91832dde42c7d17323be084cfaf67cc98de8535
Reviewed-on: https://review.typo3.org/54035


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>