[TASK] Folder not writeable should be an "Error" not a "Warning"

The Install Tool checks for permissions in it's folder structure.  A
folder (typo3temp, uploads, fileadmin) not being writeable now is an
Error and not only a Warning thus displaying a red badge in the Install
Tool main menu.

Resolves: #52407
Releases: 6.2
Change-Id: I6de5f2eace4ecdf089848efbf063a82302aea093
Reviewed-on: https://review.typo3.org/24180
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters