reevaluate force-guest-format errors

[slingamn]

• Figure out (document?) how registration works under force-guest-format
• Trying to register a guest nickname (with NS REGISTER? with REGISTER?) can produce an error. What is this error? What FAIL code does it produce?

[slingamn]

REGISTER is missing the equivalent of this code from NS REGISTER:

ergo/irc/nickserv.go

Lines 1008 to 1015 in 9a186f8

	if config.Accounts.NickReservation.ForceGuestFormat {
	matches := config.Accounts.NickReservation.guestRegexp.FindStringSubmatch(account)
	if matches == nil || len(matches) < 2 {
	service.Notice(rb, client.t("Erroneous nickname"))
	return
	}
	account = matches[1]
	}