Allow Graylog to notify if a Collector is failing

## Expected Behavior
Currently the only way to notice if a collector is failing is to view the Collector page. To aid in swift intervention on a failing collector could either or both of the following be implemented:
A dashboard widget showing any failing collectors
A new condition that allows a notification to be sent detailling the failling node.

## Current Behavior
Currently the only way to notice if a collector is failing is to view the Collector page.

## Context
Collectors can be failing for some time before someone checks the page and restarts the service.

## Moon on a Stick Behaviour
If I had to ask what the best possible feature would be then it would be Graylog identifying the failing collector and then issuing a restart command to the Sidecar service with no intervention neccessarry, only alerting if that then failed.

## Your Environment
2.4.0-beta.2+2364772 OVA (I know - we will move to a full production installation soon)