Print error message when attempting to create database extension as non-superuser
This is a follow-up for omnibus-gitlab#5612 (comment 405174147).
Creating a database extension requires superuser privileges. For installations with a non-omnibus managed postgres, this has to be done manually.
If the extension isn't installed prior to running migrations (and a non-superuser database user is being used), the migration fails with permission errors.
In order to make this more helpful, we should output a helpful error message in case the extension doesn't exist yet and the database user is not privileged enough to install it.
Additionally, we may want to add a page to documentation explaining this in more detail.