if only "[issue]" is specified the mailgw crashes.

patch mailgw.py line 273
        if not nodeid and not title:
            raise MailUsageError, '''
I cannot match your message to a node in the database - you need to either
supply a full node identifier (with number, eg "[issue123]" or keep the
previous subject title intact so I can match that.

Subject was: "%s"
'''%(classname, subject)

only (subject) is needed.