Getting the following failure when provisioning drupal_db_install,

To start over, you must empty your existing database.

Conclusion

Turns out there was a very different real exception before the message gets overwritten.

SQLSTATE[HY000] [2002] Can't connect to local MySQL server through socket '/dev/null'

I never figured out how or why we would need to set the socket file, so this workaround uses a TCP connection instead:
https://gerrit.wikimedia.org/r/#/c/234033/