refactoring: update Task response data structure (#439)

* enforce message and error in case of status False, issue 351

* refactoring _task_result function to address the improvements of task 351

* removing status argument from _task_result to better check error validation

* In discussion with @kairoaraujo, we identified an opportunity to
enhance the consistency of Task Results in our codebase.

Previously, the Celery Task result for `FAILURE` was structured as a
dictionary with keys `task_id`, `state` and `result`. The `result`
field was defined as either a `str` or `Dict[str, any]`, causing
inconsistency.

This commit introduces these changes to ensure a consistent data
structure and aligns with the specified API documentation.

For more details, refer to the discussions in this pull request
/pull/439

docs/source/devel/repository_service_tuf_worker.rst

@@ -29,6 +29,14 @@ repository\_service\_tuf\_worker.repository module
    :undoc-members:
    :show-inheritance:
 
+repository\_service\_tuf\_worker.signer module
+----------------------------------------------
+
+.. automodule:: repository_service_tuf_worker.signer
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
 Module contents
 ---------------