class SalesFormatError(TypeError):
    pass


def aggregate(sales_log):
    if not isinstance(sales_log, list):
        raise SalesFormatError('Sales log should be list')

    ...

    if not isinstance(log_record, dict):
        raise SalesFormatError('Sales record should be dict')

    ...


sales_log = read_json('path/to/file')

try:
    result = aggregate(sales_log)
except SalesFormatError:
    exit("Data file format error. Check README.md")