To fallback on decode_string

To avoid getting:

Traceback (most recent call last):
  File "nopriv.py", line 781, in <module>
    backup_mails_to_html_from_local_maildir(folder)
  File "nopriv.py", line 736, in backup_mails_to_html_from_local_maildir
    mail_subject_encoding = mail_subject_encoding,
  File "nopriv.py", line 395, in addMailToOverviewPage
    mail_subject = cgi.escape(unicode(mail_subject, mail_subject_encoding)).encode('ascii', 'xmlcharrefreplace')
UnicodeDecodeError: 'utf8' codec can't decode byte 0xe9 in position 3: invalid continuation byte