Error message fix when organizations disabled

When :oragnizations_enable in foreman/config/settings.yaml is set to false, we get corresponding error message instead of 'undefined method for nil:NilClass'

cc @pitr-ch 
Here's the rubocop (our style checker) failure: http://ci.theforeman.org/job/test_katello_rubocop/4612/checkstyleResult/new/