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.

There's still an open comment on this MR.

Sorry about that, forgot to allow port 25 on the new instance I just setup, so I wasn't getting notifications!

merged

mentioned in commit 14ce0ff7

mentioned in commit d4121e14