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.