Add metrics for images failing version-check

[MPV]

Today version-checker keeps logging when it fails on some image.

Wouldn't it be nice having a metric for this?

Maybe something like version_checker_image_failed_count?
(with labels similar to the version_checker_is_latest_version metric — such as; container, image, exported_namespace, exported_pod)

Then you could be alerted when there's images that fail the version-check. That would prompt you to check the logs for further details.

[davidcollom]

Closing as I think this is a duplicate of #82.