[BUGFIX] Notify about wrong backend user group setting

An editor can't edit a plugin as long as the page content type "Insert
Plugin" isn't allowed for the backend user group even if the access to
a plugin is defined. This patch adds a notification about missing
configuration to improve usability for integrators.

Releases: master
Resolves: #61559
Change-Id: Id7c56c2514ec4525f731c101e8e6e7dbade274e1
Reviewed-on: http://review.typo3.org/41116


Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>