Async replication: how to get an alert on failure

I have few PostgreSQL servers established with replication.
Now, is there any way to receive an e-mail in case of error during
synchronization?
I mean, is there an script (cron?) I can run (maybe every 5 minutes)
that will check if replication is stolen, and send an e-mail in case of
failure?

Thanks,

Edson Richter