add some docs and correct improve error print

[napoleon665]

When running exchange executable in a docker container we get:

panic: route ip+net: no such network interface

goroutine 1 [running]:
github.com/robaho/go-trader/internal/exchange.startMarketData()
        /app/internal/exchange/marketdata.go:261 +0x336
github.com/robaho/go-trader/internal/exchange.(*exchange).Start(...)
        /app/internal/exchange/exchange.go:284
main.main()
        /app/cmd/exchange/main.go:69 +0x517

This is due to lo0 not being available.

[robaho]

I think this might be better as:

panic(fmt.Errorf("unable to find multicast interface %q: %w",intf,err))