Raise exception when Geo secondary database is not configured

This is used by ActiveRecord before establishing a connection.
Otherwise the user would receive a crypt `relation "xxx" does not exist"
exception.

app/models/geo/base_registry.rb

@@ -4,4 +4,10 @@ class Geo::BaseRegistry < ActiveRecord::Base
   if Gitlab::Geo.secondary_role_enabled? && Gitlab::Geo.geo_database_configured?
     establish_connection Rails.configuration.geo_database
   end
+
+  def self.connection
+    raise 'Geo secondary database is not configured' unless Gitlab::Geo.geo_database_configured?
+
+    super
+  end
 end