ignore error when database exists

Let say you want to add a site+database to an existing homestead vm.

If you re-provision homestead, the create-postgres.sh script fails because existing databases, well, already exist. One solution would be to always drop databases and re-creating them. But I think thats a bit over the top because that would mess with other apps on your homestead vm, while all you wanted to do was add a site.

This small patch just ignores any postgres errors on createdb.