Avoid panic when "auto-binding" failed

[nighca]

spx tries to do auto-binding for sprites & sounds when the game starts running:

spx/game.go

Lines 256 to 271 in 1b3a5bf

	for i, n := 0, v.NumField(); i < n; i++ {
	name, val := getFieldPtrOrAlloc(v, i)
	switch fld := val.(type) {
	case *Sound:
	media, err := g.loadSound(name)
	if err != nil {
	panic(err)
	}
	*fld = media
	case Spriter:
	if err := g.loadSprite(fld, name, v); err != nil {
	panic(err)
	}
	// p.sprs[name] = fld (has been set by loadSprite)
	}
	}

It offers convenience when the user need to access some sprite or sound instance. However it works only when the variable names matches the sprite or sound name.

Within the example project (Archive.zip), it is OK to declare variables like

var (
    NiuXiaoQi NiuXiaoQi
    explosion Sound
)

While it panics if we declare variables like

var (
    myNiuXiaoQi NiuXiaoQi
    currentSound Sound
)

Sometimes it is fair to define some variables to hold sprite or sound instance during the game, which are for specific purpose that we do want to name them with some other names. That should not cause panic.

This issue proposes to remove the panic behavior when loadSound / loadSprite failed because of name mismatch here.

[nighca]

closed by #336