It is not that it's not stored that creates the problem but the fact that it is not being considered as a valid item anymore so you can't trigger anything out of it. There is a plenty of related bug reports related to it. AFAIk your options:
- Adjust the script to return a numeric value in case of errors
- Use 2.2's "internal events" to detect the "becomes unsupported" event
- Make and external script that will query the database directly and will notify you on its own or triggering a condition through zabbix's trappers