Wrap CREATE TABLE updates in if db_table_exists()

This is another one of those kind of "polish" sort of issues, but would be nice.

Because the update system doesn't allow you to run updates individually, but rather "everything from point X and forwards", if an update fails for some reason and a user tries to update again, they will get a huge page of scary, reddish-pink errors telling them that "CREATE TABLE blah.." queries all FAILED.

Let's wrap all CREATE TABLE updates in an if (!db_table_exists('blah')) block so that doesn't happen.