Description of the problem: 
Sending a notification to a user or a group that doesn't exist does not return any errors. This is an issue for MTA, for example - the workflow completes correctly even though no notification was generated. I would expect the API to return an error code that would be propagated to the workflow.

How reproducible: 100%

Steps to reproduce:

1. deploy orchestrator 1.3.0

2. Send notification to a group or user that doesn't exist

Actual results: No notification is generated, but no error code is returned.

Expected results: Notification plugin should return an error code, and workflows like MTA and M2K should register that the notification wasn't sent in a clear way to the user.