Fix bug #9888 my checking for db_info table

Darktable 1.4.1 will happily open a darktable 1.5+ database and then spew a bunch of errors. This has a risk  of database corruption. darktable 1.5 has a much fancier db versioning scheme but for 1.4.x all we need to do is check if the db_info table exists and if it does abort as we know we're in a 1.5+ database.