get a cell, if it does not exist fail
                note that column at row START AT 1 same as excel

def GetCellValueNoFail (self, column):
                """ get a cell, if it does not exist fail
                note that column at row START AT 1 same as excel
                """
                cell = GetCellValue(self, column)
                if cell == None:
                        raise ValueError("cell %d does not exist" % (column,))
                return cell