[feat]: Return user-facing errors to taskproc where it makes sense

[kyteinsky]

See nextcloud/integration_openai#296
Although changes in nc_py_api might be required.

Cases where this would make sense:

• when no docs are found for a scoped query:

  context_chat_backend/context_chat_backend/task_fetcher.py

  Lines 546 to 548 in 0d3308f

  	except ContextException as e:
  	LOGGER.warning(f'Context error for task {task["id"]}: {e}')
  	return_error_to_nextcloud(task['id'], e)