[UX] Check for missing modules during core update

Describe your issue or idea

This is a follow-up to #2842 where we realized that a lot of people were upgrading from Drupal to Backdrop in a way that will result in broken sites.

The root of the problem, it seems, is that people want to update core before contrib, and contrib modules often require their updates to be run between two core updates.

One way we might mitigate this problem is to at least let people know that some modules might be missing by setting a warning at the top of the update page. We could check for missing modules by: * Check the system table for all modules. Check the modules location(s) to see which modules are present. Compare the two. * Check if there are tables in the database which are not defined by a hook_schema.