Add minor safety to DROP INDEX/TABLE statements in migrations

migrations/11_add_reserved_ids_table_down.sql

migrations/13_add_export_tables_down.sql

migrations/16_add_media_attributes_table_down.sql

migrations/1_create_tables_down.sql

migrations/20_create_id_hold_table_down.sql

migrations/2_add_animated_col_down.sql

migrations/2_add_animated_col_up.sql

migrations/6_track_last_accessed_times_down.sql

migrations/7_add_datastore_down.sql