Invalidate cache for featured asset URL

When a story's featured assets are updated, invalidate the cached
URL.
More robust error handling/reporting

Use ``storybase.messaging.management`` for the logger to allow
for more fine-grained handling of these logs (since the management
command is meant to run frequently from cron, the logs can get
pretty verbose).

Catch all exceptions and log errors so the command doesn't die
inexplicably.

Addresses #580