Forward lc0's stderr to lc0-client's stderr

[mooskagh]

It would be nice to show entire lc0 stderr output when running client, for example by displaying it on lc0-client's stderr. That way when something is wrong with lc0 it can be seen.

[jnewlin12345]

It already does this:

go func() {
		stderrScanner := bufio.NewScanner(stderr)
		for stderrScanner.Scan() {
			fmt.Printf("%s\n", stderrScanner.Text())
		}
	}()

[mooskagh]

That's weird because I don't see things like that:

       _
|   _ | |
|_ |_ |_| built Jun  9 2018

in lc0-client's output.

Also maybe instead of doing that manually, there's a way to do that automatically? (in python it's certainly possible to somehow share stderr with subprocess when creating one).

[jnewlin12345]

are you on windows? I see those lines in linux

[Tilps]

I submitted a PR to hopefully reduce the potential loss of such messages (which are definitely working on windows) by connecting lc0 stderr directly to stdout of client rather than using the goroutine.
If that doesn't work, maybe we should try connecting it to stderr instead.