Explain missing Geo registry tables in geo:status

What does this MR do and why?

gitlab-rake geo:status builds a GeoNodeStatus, which counts every registry table in the tracking database. When the Geo migrations were never run or stopped part way, the task died with a raw PG::UndefinedTable: relation "ci_secure_file_registry" does not exist for whichever registry table came first, giving the administrator nothing to act on.

This MR makes gitlab:geo:status check for pending tracking database migrations on a secondary before it builds the status. When migrations are pending, the task aborts with the db:migrate:status:geo and db:migrate:geo commands and a link to the Geo troubleshooting page.

The issue also suggests a documentation update; I have kept this MR to the code change and can follow up on the docs separately if the maintainers want a dedicated section.

References

• Resolves #565971 (closed)

Differences

Before: On a secondary whose tracking database is missing registry tables, gitlab-rake geo:status raises ActiveRecord::StatementInvalid: PG::UndefinedTable.

After: The task aborts with: "The Geo tracking database is missing these registry tables: ci_secure_file_registry" followed by the two rake commands to run and the docs link.

How to set up and validate locally

1. On a Geo secondary (or in a Rails console with a tracking database), drop one registry table, for example Geo::TrackingBase.connection.drop_table(:ci_secure_file_registry).
2. Run bundle exec rake geo:status.
3. On master it raises; with this MR it prints the actionable message and exits 1.

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