Use common exception hierarchy for all drivers

[spalladino]

Ensure that exceptions thrown from https://github.com/bcardiff/crystal-pg/blob/feature/db-0.3.0/src/pq/error.cr, for instance, extend from DB::Error

[bcardiff]

~~Also ensure IO::EOFError or connection lost is raised as DB::ConnectionLost for pool retry logic in crystal-pg. (check if PQ::ConnectionError should inherit DB::ConnectionLost)~~ done