Feature request: Fallback to local snapshot when failing to upload external snapshot.

Current behaviour: when an Actor has configured `SnapshotType_SNAPSHOT_TYPE_EXTERNAL` but substrate fails to upload the external snapshot after checkpointg, we have to set the Actor to CRASHED state, because the most recent snapshot is lost.

Feature request: if we can fallback to storing the snapshot locally and upload later, we won't have to set the Actor to CRASHED state right away.