Error stats: add a count by column and code

Implementation of #12 (closed). Add more things to error stats: a count of error codes by column.

    {
        'count': 12,
        'structure-errors': {
            'count': 1,
            'count-by-code': {
                'invalid-column-delimiter': 1
            }
        },
        'value-errors': {
            'count': 10,
            'rows-count': 3,
            'count-by-code': {
                'type-or-format-error': 2,
                'pattern-constraint': 7,
                'french-siret-value': 1,
            },
            'count-by-col-and-code': {
                'A': {'required-constraint': 7},
                'B': {'type-or-format-error': 3}
            }
        },
    }