Service panics when connection to camera fails

[lenny-goodell]

Start service with default configuration and devices. The following panic will occur.

level=ERROR ts=2021-06-30T17:15:19.7473004Z app=device-camera source=onvifclient.go:52 msg="Error initializing ONVIF Client: %!w(*url.Error=&{Post http://192.168.2.105/onvif/device_service 0xc0000b22e8})"
panic: interface conversion: interface {} is nil, not string