re #4525 correctly copy default

git-svn-id: svn://develop.studip.de/studip/trunk@31568 c5b85792-5220-0410-ad23-ed5b6b8ad999
fixes #4660
show error message and prevent form submitting if no recipients selected

git-svn-id: svn://develop.studip.de/studip/trunk@31569 c5b85792-5220-0410-ad23-ed5b6b8ad999