parsemail: Fix return value for find_content error case

parsemail: Fix return value for find_content error case

If we fail to decode a message payload, we'll fail with the following:

  Traceback (most recent call last):
    File "./patchwork/bin/parsemail.py", line 563, in <module>
      sys.exit(main(sys.argv))
    File "./patchwork/bin/parsemail.py", line 553, in main
      return parse_mail(mail, args['list_id'])
    File "./patchwork/bin/parsemail.py", line 464, in parse_mail
      (patch, comment, filenames) = find_content(project, mail)
  ValueError: need more than 2 values to unpack

- as the error condition for find_content only returns a 2-tuple. This
change fixes the error case to the 3-tuple return type.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Stephen Finucane <stephen.finucane@intel.com>