cubicweb #229661 handle errors on sending email [open]

When a notification email is sent and doesn't get through (for example the STMP server refuses it because he thinks it's spam), cubicweb should handle the error and retry later or show the error to the site admin.

For example

2009-03-09 09:59:54 - (cubicweb.session) ERROR: error sending mail to [u''] ((550, '5.7.1 Blocked by SpamAssassin'))
Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/cubicweb/server/", line 85, in sendmails
    smtp.sendmail(heloaddr, recipients, msg.as_string())
  File "/usr/lib/python2.5/", line 707, in sendmail
    raise SMTPDataError(code, resp)
SMTPDataError: (550, '5.7.1 Blocked by SpamAssassin')
done in<not specified>
load left0.500
closed by<not specified>