#21 Alerts trigger upon non-threshold failures

[jkraybill]

If you have a threshold alert, but some other failure happens, e.g. a database connection is refused, the listener still sends an email, with no body. This is confusing because the email will have a subject like "Unusually high number recorded" with no body, but what actually happened was an exception during reporting.

The desired behavior should be that these threshold listeners do not fire at all upon error; instead the user should be able to configure a separate error/exception listener for some (or all) reports.