FATAL artnet discovery errors

[SpComb]

The qmsk-dmx process exits if sending ArtNet discovery packets fails:

INFO              qmsk.web:                               POST presets/test: &heads.APIPresetParams{preset:(*heads.Preset)(0xc82001f880), Intensity:(*heads.Intensity)(0xc82026ea20)}
WARNING               artnet:controller<[::]:6454>          discovery timeout: 192.168.2.100:6454
INFO                                                      artnet.Discovery: 192.168.2.100:6454:
INFO                                                            Name: NodeMCU-ARTNET
INFO                                                            Output 0: 0:0.1
WARNING               artnet:controller<[::]:6454>          discovery timeout: 192.168.2.100:6454
INFO                                                      artnet.Discovery: 192.168.2.100:6454:
INFO                                                            Name: NodeMCU-ARTNET
INFO                                                            Output 0: 0:0.1
WARNING               artnet:controller<[::]:6454>          discovery timeout: 192.168.2.100:6454
INFO                                                      artnet.Discovery: 192.168.2.100:6454:
INFO                                                            Name: NodeMCU-ARTNET
INFO                                                            Output 0: 0:0.1
WARNING               artnet:controller<[::]:6454>          discovery timeout: 192.168.2.100:6454
INFO                                                      artnet.Discovery: 192.168.2.100:6454:
INFO                                                            Name: NodeMCU-ARTNET
INFO                                                            Output 0: 0:0.1
WARNING               artnet:controller<[::]:6454>          discovery timeout: 192.168.2.100:6454
WARNING               artnet:universe<0:0.1>                SendDMX: SendDMX broadcast 0:0.1: write udp [::]:6454->192.168.2.100:6454: sendto: network is unreachable
ERROR               artnet:controller<[::]:6454>          discovery: sendPoll: write udp [::]:6454->192.168.2.100:6454: sendto: network is unreachable
FATAL                                                     artnet discovery ended

This happens if the interface associated with the artnet discovery addresse goes link-down. The controller should keep running instead.