Account for the 250 request/minute rate limit

Content was being unpublished when using the Acquia DAM Asset Importer → module to synchronize the canonical asset repository with Drupal.
Investigation uncovered an undocumented rate limit of 250 requests per minute that returns a 404 (?!) with the message "Not Authorized".

Ideally the service would return a better code and response (here's a list of great implementations). Barring that, the module should account for the rate limit and not unpublish assets when it is exeeded.