FIX: prevents PM to large groups (#28681)

This commit introduces a new hidden site setting: `group_pm_user_limit`, default to `1000` which will raise an error when attempting to create a PM target a large group.

lib/has_errors.rb

@@ -15,14 +15,14 @@ module HasErrors
     false
   end
 
-  def rollback_with!(obj, error)
-    obj.errors.add(:base, error)
+  def rollback_with!(obj, error, **kwargs)
+    obj.errors.add(:base, error, **kwargs)
     rollback_from_errors!(obj)
   end
 
   def rollback_from_errors!(obj)
     add_errors_from(obj)
-    raise ActiveRecord::Rollback.new
+    raise ActiveRecord::Rollback.new, obj.errors.full_messages.join("\n")
   end
 
   def add_error(msg)