When executing in Web UI:

{ 
  "method":"hbacrule_mod",
  "params":[["aa"],{"all":true,"rights":true,"usercategory":"all"
}

And user category contained some user and group. Following result was returned to Web UI.

{
   "error": "Gettext(\"user category cannot be set to 'all' while there are allowed users\", domain='ipa', localedir=None)"
}

The error attribute doesn't contain message itself but a python call which should get the message. Fortunately the call text contains the message so user can understand something.

Error attribute should contain only the error message itself.