strings as exceptions are invalid in python2.6

There are still few exceptions in Mailman/Errors.py not converted to classes. In python 2.6 strings as exceptions are no longer supported.

The errors in mailman log look like this:
admin(31385): Traceback (most recent call last):
admin(31385): File "/usr/lib64/mailman/scripts/driver", line 101, in run_main
admin(31385): main()
admin(31385): File "usr/lib64/mailman/Mailman/Cgi/subscribe.py", line 96, in main
admin(31385): File "usr/lib64/mailman/Mailman/Cgi/subscribe.py", line 176, in process_form
admin(31385): File "usr/lib64/mailman/Mailman/MailList.py", line 917, in AddMember
admin(31385): TypeError: exceptions must be classes or instances, not str