Back-off failed image metadata fetches

[squaremo]

It's possible to have a situation in which flux is trying to fetch image metadata to which it will never have access. For example, if you preload images on your hosts, rather than pulling them from a registry.

To reduce the noise in the logs, it'd be good to have a back-off for fetching image metadata that fails repeatedly.

[squaremo]

The warmer (scanner) rewrite in #1354 does backoff in general, including setting a longish period if the fetch gave an error.