Handle unavailable system info

What does this MR do?

Handle the case where we can't get system info without blowing up. As this is the first tab in the monitoring section, it's difficult to get to the other tabs if this page throws a 500. Also be more specific about the info we want, so we don't fail on something we don't care about (like /proc/net/dev).

Are there points in the code the reviewer needs to double check?

Don't think so.

Why was this MR needed?

grsecurity can prevent users from reading /proc, which is what Vmstat uses to find CPU and memory info.

What are the relevant issue numbers?

Closes #21378 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together