In `wpdb::db_connect()`, allow the loading of a custom database error template...

In `wpdb::db_connect()`, allow the loading of a custom database error template - this is already allowed in `dead_db()`.

In `dead_db()`, move the call to `wp_load_translations_early()` before the inclusion of the `db-error.php` file to allow translation in both locations before the template is loaded.

Props sbruner, kovshenin.
Fixes #25703.


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