returns a 501 instead of exception if app is not installed - #13088
returns a 501 instead of exception if app is not installed - #13088

Signed-off-by: Maxence Lange <3575ce786d1bd5fcab86496f54298b45243a16d4@artificial-owl.com>

nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,nextcloud/server