[FEATURE] ext:install Show number of folder structure errors in menu

Similar to the environment status the number of folder structure
errors are now shown in the left menu.

Change-Id: I665c32e8a3c4389941437a2bb6caeb32b862b673
Resolves: #52368
Releases: 6.2
Reviewed-on: https://review.typo3.org/24136
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn