Handle notifier errors; explicit failure for webhook notifiers

[gg-mmill]

Context

Right now, if one notifier raises, the program will stop (i.e. further notifiers will not be triggered).
Also webhook notifiers fail silently

DoD

• if one notifier fails:
  • handle and log the exception/error
  • return non-null error code at the end of the program (so that when looking at cloudwatch monitoring, we can see there are issues)
• Raise when webhook notifier did not perform a successful call.