bsdinstall: Add quotes around error message argument

When error is called with a message with spaces (and probably multiple
lines) these are passed into dialog unquoted and an error message was
presented, wrap with quotes.

Reviewed by:	bapt, allanjude
Sponsored by:   Ampere Computing LLC
Sponsored by:   Klara Inc.
Differential Revision:	https://reviews.freebsd.org/D33918

usr.sbin/bsdinstall/scripts/fetchmissingdists

@@ -30,7 +30,7 @@
 
 error()
 {
-	dialog --backtitle "FreeBSD Installer" --title "Error" --msgbox $1 0 0
+	dialog --backtitle "FreeBSD Installer" --title "Error" --msgbox "$1" 0 0
 	exit 1
 }