Clarify error about incomplete traceability_relationship_to_string

[JasperCraeghs]

When configuration variable traceability_relationship_to_string is incomplete, the doc build is exited with exception: Relationships {'<<relationship_name>>'} are missing from configuration variable 'traceability_relationship_to_string'.

If this is check for completeness were to be removed, a warning would be reported when this relationship is used: Traceability: relation <<relationship_name>> cannot be translated to string. However, this warning is bugged and would crash the doc build with the following error:

Traceback (most recent call last):
  File "/opt/venv/lib/python3.11/site-packages/sphinx/events.py", line 96, in emit
    results.append(listener.handler(self.app, *args))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.11/site-packages/mlx/traceability.py", line 249, in process_item_nodes
    node.perform_replacement(app, env.traceability_collection)
  File "/opt/venv/lib/python3.11/site-packages/mlx/directives/item_directive.py", line 28, in perform_replacement
    self._process_relationships(dl_node, app)
  File "/opt/venv/lib/python3.11/site-packages/mlx/directives/item_directive.py", line 65, in _process_relationships
    self._list_targets_for_relation(rel, targets, *args)
  File "/opt/venv/lib/python3.11/site-packages/mlx/directives/item_directive.py", line 84, in _list_targets_for_relation
    env.docname, self.line)
    ^^^^^^^^^^^
  File "/opt/venv/lib/python3.11/site-packages/sphinx/environment/__init__.py", line 534, in docname
    return self.temp_data['docname']
           ~~~~~~~~~~~~~~^^^^^^^^^^^
KeyError: 'docname'

[Letme]

Can you add a test case that failed as well?

[JasperCraeghs]

Can you add a test case that failed as well?

This warning should no longer be used thanks to the error handling I implemented on master branch in May. 76f8270
I added a test case.