log notifications and returnvalue locally

## Current Behavior
When Graylog fires a notification (http for example) no option is given to see if the event has triggered the notification and what (if any) return value was seen.

Should the receiver have a problem or is not working Graylog does not know that and if you debug a pipeline of different services that play together you can't see if Graylog has acted as it should.

## Your Environment

* Graylog Version: 3.1

[z#842864]