fix logged error messages to be more accurate

* The error logged on failure to load an org's "users" group during
  the association process is add_user_to_usag_failed, which is
  incorrect. This change updates it to be accurate and meaningful:
  'fetch_org_users_group_by_name_failed'
* Admin user deletion is blocked - but the error message logged was
  incorrectly specified as 'cannot_dissociate_self_while_admin'.  Changed
  it to 'admin_user_removal_not_permitted'

CI running: http://wilson.ci.opscode.us/job/chef-server-12-build/306/

ping @opscode/server-team