[TASK] Display count of failed error checks in install tool menu

With displaying the count of failed error checks in the navigation menu
(like the count of unread messages in an email inbox) a user will more
likely enter the "System environment" section of the install tool and
take care about the found errors and warnings.

Change-Id: I3d9b628b428da13992c3dc1e7e5ab6beaeda5ce1
Resolves: #51410
Releases: 6.2
Reviewed-on: https://review.typo3.org/23380
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters