[Discover] Refactor on_save_search to improve error handling

[davismcphee]

Currently the approach on_save_search uses includes modifying the search object in place, attempting to save, and rolling back the modified properties on failure.

This approach is brittle since it's easy to forget to roll back the properties on failure. Instead we should clone the object before changing the properties, attempt the save, and only update the original search on success.

extracting values of the given savedSearch, mutating the object, continue to mutate it further when preparing to save, and if it fails, we unset values ... we should better provide a saved search for saving, and if it's successful we continue operations to modify / change our saved search, rather than undoing changes

Originally posted by @kertal in #136162 (comment)

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)