Healthcheck shouldn't report error on missing status response

[karol-kokoszka]

As per title

https://github.com/scylladb/scylla-manager/blob/bff2401fa39d04dfd0592524d301816ba59149db/pkg/service/healthcheck/runner.go#L68-L80

Service should save the last live nodes and rely on them if it didn't manage to update this information.