Site online with db offline

How are you loading the secondary databases? What is "the usual error message"?

As far as I can tell, if you have db_debug set to false in your database config, as long as the database library doesn't find some issue with the other configuration values, you should be able to continue if the database fails to connect, as long as you don't try to use the failed database connection.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide