attachments / featured images not fault tolerant if missing

Problem/Motivation

When testing various migrations. It is too easy to jam up when the file attachments are not present at the URLs in the XML file. (whether 404, server is missing, etc)

This occurs when the import imagefield is selected but images cannot be loaded by url.

Steps to reproduce

Try to import anything with file attachment / featured image with unavailable URL

Proposed resolution

skip on error for missing file attachment / image URLs.

i think this should be configurable ideally, have timeout and so on.

Remaining tasks

Not certain. Maybe, if the image field is not selected, do not create the attachments migration at all, there is no if statement here. Unlike the tag_vocabulary right below it. If so, then it would need to not be a dependency for content post and content page.

    // Set up the attachment migration.
    $this->attachmentID = $this->configuration['prefix'] . 'wordpress_attachments';
    $migration = $this->createEntityFromPlugin('wordpress_attachments', $this->attachmentID);
    $migration->set('migration_group', $this->configuration['group_id']);
    $process = $migration->get('process');
    $process['uid'] = $this->uidMapping;
    $migration->set('process', $process);
    $migration->save();

User interface changes

Improve the UI message. perhaps as a placeholder until this is improved.

API changes

Data model changes