Improve exception message when not a secondary

What does this MR do?

This MR improves the message when exception Gitlab::Geo::GitPushSSHProxy::MustBeASecondaryNode is raised from:

Node is not a secondary

to

Node is not a secondary or there is no primary Geo node

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6456#note_99916236

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?