DBForge create_database if not exists

Like `create_table`, the `create_database` method should have `$exists = false`
Such that:
`create_database("name", true)` will render a "CREATE DATABASE `name` IF NOT EXISTS"
and in case it returns an error, will NOT call "show_error" (which exists the page)

This is changing just a few lines of code, with not DB engine compatibility issue to my knowledge