Problem/Motivation
All database specific code belongs in the mysql, pgsql or sqlite module. Just like contrib database driver module need to do. The core database driver modules are the example for the contrib ones.
In the class Drupal\user\UserStorage, there is a MySQL exception which should be moved to the mysql module.
Proposed resolution
Move the MySQL exception in UserStorage to the mysql module.
Remaining tasks
TBD
User interface changes
None
API changes
None
Data model changes
None
Release notes snippet
TBD