Some of exceptions thrown by route enhancer should not be ignored

Problem/Motivation

If Drupal fails to write cache or something in a certain point, Page Manager returns 404 page, instead of 5xx error.
It is because that \Drupal\page_manager\Routing\VariantRouteFilter catches and ignores all \Exceptions thrown by route enhancers.

try {
  $attributes = $enhancer->enhance($attributes, $request);
}
catch (\Exception $e) {
  return FALSE;
}

DB errors should be reported as 5xx errors, not Not Found. This can be a serious problem if the site has a cache server. In our case, the cache server cached 404 responses, and then end-users could not access some existing pages. If 5xx status codes had been returned, it would have been just temporal errors due to high load.

Steps to reproduce

1. Install Drupal 9.3.
2. Install Page Manager 8.x-4.0-beta6.
3. Create a Page and a Page Variant.
4. Implement a custom route enhancer which internally throws \Drupal\Core\Database\DatabaseException. (TODO: I will add such classes as test cases.)
5. Access the page created at step 3.
6. 404 Not Found

Proposed resolution

Catch and re-throw \PDOException, DatabaseException and TempStoreException thrown by route enhancers.

Remaining tasks

• Add tests
• Review patch

User interface changes

None

API changes

None

Data model changes

None