It appears, however, that this functionality is not supported in 2.2beta!
I discovered this "regression" after successfully running the installer to upgrade a copy of a 1.12.1 install on a test machine. As luck would have it, while attempting to view the converted site, an unrelated MySQL corruption trashed some db user passwords, resulting in login failures. I noticed the MySQL problem because visiting the site yielded an all white screen with this generic message:
Not, however, the customized db_error.html page that I would have expected/desired.Sorry, something has gone wrong. Please contact a site administrtor. (CMSMS\Database\DatabaseConnectionException)
You can reproduce the same error by temporarily changing a db_username or db_password value in config.php and then attempt to view any page on your site.
I don't know if the loss of customization was inadvertent or by design, but it would be great if db_error.html or some other form of DB error reporting customization were available in 2.2.
Thanks to the team for all the effort!
Now, back to the upgrade testing...