Run readiness checker during updater service

Problem/Motivation

Right now we are actually calling the readiness checkers when \Drupal\automatic_updates\Updater::begin() is called.

Proposed resolution

We should call the readiness checker and throw an exception if there are any errors.

Remaining tasks

Make a follow up issues for:

1. Calling the checkers in \Drupal\automatic_updates\Form\UpdaterForm::buildForm() to not give the option to start an update if there are error and display warnings
2. Discuss switching from the current checker system to event subscriber system. This would allow checkers to run at different stages of the update. For instance checking that a staged update is correct and didn't update any modules that were not intended. see #3179266: Automatic Updates that require updates to contrib and custom extensions should be rejected and #3222608: Check for known security advisories on staged Automatic Updates

User interface changes

None

API changes

Data model changes