Naming and location of database exceptions

Spin off from #3622582: Cleanup database exceptions.

Problem/Motivation

Database exception classes are scattered across multiple namespaces/folders and that is a bit confusing.

Also, DatabaseExceptionWrapper is a bit too generic in name, as it and its descendants in fact are triggered once the lower level database client has to return errors to callers, and that's specific of the roundtrip of SQL statements between Drupal and its underlying db layer.

Proposed resolution

1. Move all database exceptions to the Exception namespace. For BC, keep the original classes as aliases in the old location for a major's cycle
2. Rename DatabaseExceptionWrapper to ClientConnectionException, with the same BC logic.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet