When testing one of modifications for Opencart it was found that default modification.php of Opencart 2.0.x and 2.1.x processes incorrect the attribute error="skip" of tag . It breaks processing instead skipping.
Open default file  admin/controller/extension/modification.php and find:

// Skip current operation
if ($error == 'skip') {
	break;
}

Replace it to:

// Skip current operation
if ($error == 'skip') {
    $log[] = 'SKIP!'; // info to log
    continue; // to next iteration
}

The attribute error="skip" will be processed correct. Actual for Opencart 2.0.x and 2.1.x.