network_manager: Cleanup VLAN creation
network_manager: Cleanup VLAN creation

This adds proper error handling to the path so that we don't segfault.

Change-Id: I65f39844bd36d9f5f97df33a64f5d07e767dd48c
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/phosphor-networkd,openbmc/phosphor-networkd