Geo: Gracefully catch incorrect db key on primary

What does this MR do?

When the primary isn't using the correct db_key_base, the /admin/geo_nodes should gracefully show this exception.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests
  • [-] Added for this feature/bug
  • [-] All builds are passing
• [-] Conform by the merge request performance guides
• [-] Conform by the style guides
• [-] Branch has no merge conflicts with master (if it does - rebase it please)
• [-] Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ee#2869.