tidy up nogroups error string

Fix #513

lang/en/attendance.php

@@ -338,7 +338,7 @@ $string['noautomark'] = 'Disabled';
 $string['nocapabilitytotakethisattendance'] = 'You tried to change the attendance of a session with the cmid: {$a} that you do not have permission to modify.';
 $string['nodescription'] = 'Regular class session';
 $string['noeventstoreset'] = 'There are no calendar events that require an update.';
-$string['nogroups'] = 'You can\'t add group sessions. No groups exists in course.';
+$string['nogroups'] = 'This activity has been set to use groups, but no groups exist in the course.';
 $string['noguest'] = 'Guest can\'t see attendance';
 $string['noofdaysabsent'] = 'No of days absent';
 $string['noofdaysexcused'] = 'No of days excused';