send email when PV becomes invalid or disconnected

[prjemian]

requested by Tom Caradoc-Davies <tom.cd at synchrotron.org.au>, Australian Synchrotron

I am looking at using your pvMail package for sending email alarms based on epics PVs.
One question, the system sends emails when the value of a PV changes from 0 to 1. Does it send emails if the PV becomes invalid or disconnected (such as the IOC shutting down)?

I want a robust alarm handler but one of the failure modes is the IOC generating the alarm PV failing or being turned off.