def error_string(self):
        """Returns a well formed error message

        e.g at least 5 times but was called 4 times

        :rtype string
        """

        if self.has_correct_call_count():
            return ''

        return '{} instead of {} {} '.format(
            self._restriction_string(),
            self.count,
            pluralize('time', self.count)
        )
Returns a well formed error message

        e.g at least 5 times but was called 4 times

        :rtype string