Handle missing DBs in connect_to_db?

This ensures CurrentSettings#connect_to_db? returns "false" in the event
of a database not existing, instead of raising an error.

lib/gitlab/current_settings.rb

@@ -41,6 +41,9 @@ module Gitlab
       use_db && ActiveRecord::Base.connection.active? &&
                 !ActiveRecord::Migrator.needs_migration? &&
                 ActiveRecord::Base.connection.table_exists?('application_settings')
+
+    rescue ActiveRecord::NoDatabaseError
+      false
     end
   end
 end