feat: improve context_chat_backend error message

[kyteinsky]

Describe the feature you'd like to request

nextcloud/context_chat_backend#268

In LangRopeService.php line 168:                                                                                
Error received from Context Chat Backend (ExApp) with status code 503: Context Chat is disabled, enable it from AppAPI to use it.

also mention the deploy daemon check

Describe the solution you'd like

/

Describe alternatives you've considered

/