Use stderr for Flax.Build error messages

This turns the error messages from Flax.Build to red in Flax Editor
console output.

Source/Tools/Flax.Build/Log.cs

@@ -65,7 +65,10 @@ namespace Flax.Build
                     if (ApplyConsoleColors)
                         Console.ForegroundColor = color;
 
-                    Console.WriteLine(Indent + message);
+                    if (color != ConsoleColor.Red)
+                        Console.WriteLine(Indent + message);
+                    else
+                        Console.Error.WriteLine(Indent + message);
 
                     if (ApplyConsoleColors)
                         Console.ResetColor();