Do not fail on notify when quantum and melange are out of sync.
Do not fail on notify when quantum and melange are out of sync.

This patch adds the `ignore_missing_network_data` flag which will log
exceptions rather than raising them when generating usage exists
notifications.

Change-Id: Ie11bc9028790c2d1b3de1f4709a9e17a67e06f26

n0ano/ganttclient