Display error if user can't read requested sub

I bookmarked a few subs, and when I loaded the bookmarks later I was unexpectedly shown the group list in one of the tabs. I realized this was because I wasn't signed in and so didn't have permission to read the one sub.

This change adds a message to make it clear why the group list is being shown instead of the expected sub.

webv4/pages/001-forum.ssjs

@@ -145,6 +145,13 @@ if (typeof http_request.query.notice !== 'undefined') {
 	writeln(format(strings.notice_box, http_request.query.notice[0]));
 }
 
+if (typeof http_request.query.sub !== 'undefined' &&
+	typeof msg_area.sub[http_request.query.sub[0]] !== 'undefined' &&
+	!msg_area.sub[http_request.query.sub[0]].can_read
+) {
+	writeln('<div class="alert alert-danger">Sorry, you do not have access to the requested sub.</div>');
+}
+
 if (typeof http_request.query.sub !== 'undefined' &&
 	typeof msg_area.sub[http_request.query.sub[0]] !== 'undefined' &&
 	typeof http_request.query.thread !== 'undefined' &&