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

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