code: def group_errors(graph: BELGraph) -> Mapping[str, List[int]]:
      
      warning_summary = defaultdict(list)
  
      for _, exc, _ in graph.warnings:
          warning_summary[str(exc)].append(exc.line_number)
  
      return dict(warning_summary)
docs: Group the errors together for analysis of the most frequent error.
  
      :return: A dictionary of {error string: list of line numbers}