Changes to envoy notification mailing behaviour

- Trap failure of "ERROR ON ENVOY" mail and fallback to log Dumper
- Clear users' notification in all cases, since a problematic notification can cause the whole queue to get stuck, never clear.

The error log fallback *should* be temporary, to figure out what's actually going on with the "ERROR ON ENVOY" mail, but nothing persists like a temporary solution - need to keep an eye on it. This logs both mail errors, so we should see the problematic notification too.