situation

Actions like: Delete, Freeze, Shrink all execute ES actions like
DeleteIndex and CloseIndex that throw IllegalArgumentExceptions in the cases
that snapshots of their respective indices are in progress.

What can ES do about it?

A SnapshotInProgressException can be thrown instead of an IAE so
that ILM can be signaled more explicitly that this is the case so that
it can retry actions appropriately. Actions should not move to an error
step.