actually catch wp_insert_attachment errors

[drzraf]

wp_insert_attachment() returns 0 unless the 4th parameter is set to TRUE in order to return a WP_Error (since 4.7.0) https://core.trac.wordpress.org/browser/tags/4.9.5/src/wp-includes/post.php#L4898

[rmccue]

@drzraf Do not spam comment this repository.