Description

A customer said they had an issue recently where a syntax error in the wildcard index pattern in SLM meant that even though the snapshot was considered successful, there were no indices being snapshotted (empty snapshots).

Perhaps we should generate a warning or error message that can be alerted on when the snapshot configuration captures no data on execution. Or we should fail the Snapshot so that the SLM Health Report can report on it.

For the benefit of users for whom the correct SLM policy really is "back up all my foo_* indices, if there are any, and sometimes there aren't, and that's fine" perhaps we can couple the new error behavior with an "allow_no_indices": true option at the SLM policy level.