[ERM-12] Enrich the resulting notification with reasons why rollout failed for some pages

[smiakchilo]

After the task #22 is implemented, I, as a user of Rollout Manager, additionally want to know the reason why te rollout failed for some of my pages. This can be due to the "rollout live copies" setting disabled, or a page path added to the "excluded paths" list in a live copy, etc. The reason of failure should be put beside the path of every failed page in the list.

The implementation might depend on the diagnostic tool/method in #14 . For every failed path, we call the diagnostic method and learn the reason of failure.