Improve error message when loading audio-only disc

[YoshiRulz]

Currently discards the DiscType, only writing game.System:

BizHawk/src/BizHawk.Client.Common/RomLoader.cs

Lines 301 to 305 in 756c79f

	case DiscType.AudioDisc:
	case DiscType.UnknownCDFS:
	case DiscType.UnknownFormat:
	game.System = _config.TryGetChosenSystemForFileExt(ext, out var sysID) ? sysID : VSystemID.Raw.NULL;
	break;

So it ends up here:

BizHawk/src/BizHawk.Client.Common/RomLoader.cs

Line 412 in 756c79f

if (cores.Count == 0) throw new InvalidOperationException("No core was found to try on the game");