Added error message if color could not be adjusted

The failsafe in ColorValidator aborts after 255 iterations of
adjusting the color and picks a random color. This message makes it
clearer to the user. Results in two messages being displayed,
first one about adjusting and the about a random color pick.

OpenRA.Mods.Common/ColorValidator.cs

@@ -161,6 +161,7 @@ namespace OpenRA.Mods.Common
 				if (attempt >= 255)
 				{
 					color = RandomPresetColor(random, terrainColors, playerColors);
+					onError("Color could not be adjusted enough, a new color has been picked.");
 					break;
 				}