cronjobs: provide fallback means in case the underlying class of a task does not exist (prevents fatal errors), re #4996

git-svn-id: svn://develop.studip.de/studip/trunk@32140 c5b85792-5220-0410-ad23-ed5b6b8ad999
cronjobs: adjust tasks view so invalid tasks cannot be executed (fixes a dialog problem as well), re #4996

git-svn-id: svn://develop.studip.de/studip/trunk@32141 c5b85792-5220-0410-ad23-ed5b6b8ad999