Keep track of failed media imports and skip on next AJAX calls

[donmhico]

Description

This PR keeps track of the failed media imports via transient named ocdi_importer_data_failed_attachment_imports and skip them on the next AJAX calls to prevent an infinite loop.

Motivation

Fixes #216.

Testing procedure

1. Create an export the has multiple attachments.
2. Edit the content xml and change the attachment URLs to anything that would be invalid.
3. Use this snippet

   function ocdi_change_time_of_single_ajax_call() {
   return 1;
   }
   add_filter( 'ocdi/time_for_one_ajax_call', 'ocdi_change_time_of_single_ajax_call' );

4. Perform the OCDI import.
5. The import should complete with failed media imports.