Bad regexps in *_these_nonmembers, subscribe_auto_approval and ban_list should be logged

The list attributes *_these_nonmembers, subscribe_auto_approval and ban_list accept lists of email addresses and regexps (beginning with '^') matching email addresses. Currently, if the regexp is invalid, it is silently ignored. The GUI does not accept invalid regexp entries, but there are other ways they could be introduced. An invalid regexp should at least be logged.