Add stacktrace to logging in top-level exception-handling

[eidens]

This change makes debugging pipeline failures much easier. We go from this:

2025-09-10 15:15:31 - src.soda_curation.pipeline.assign_panel_source.assign_panel_source_openai - ERROR - Error calling OpenAI API: Request timed out.
2025-09-10 15:15:31 - __main__ - ERROR - Pipeline failed: 'NoneType' object has no attribute 'not_assigned_files'

To this:

2025-09-10 15:15:31 - src.soda_curation.pipeline.assign_panel_source.assign_panel_source_openai - ERROR - Error calling OpenAI API: Request timed out.
2025-09-10 15:15:31 - __main__ - ERROR - Pipeline failed: 'NoneType' object has no attribute 'not_assigned_files'
Traceback (most recent call last):
  File "/app/src/soda_curation/main.py", line 157, in main
    processed_figures = panel_source_assigner.assign_panel_source(
  File "/app/src/soda_curation/pipeline/assign_panel_source/assign_panel_source_base.py", line 82, in assign_panel_source
    self._assign_to_figure(figure)
  File "/app/src/soda_curation/pipeline/assign_panel_source/assign_panel_source_base.py", line 139, in _assign_to_figure
    figure, panels, assigned_files_list.not_assigned_files
AttributeError: 'NoneType' object has no attribute 'not_assigned_files'