Fix serialization of custom exceptions.

OpenRA.Game/FieldLoader.cs

@@ -16,6 +16,7 @@ using System.Drawing.Imaging;
 using System.Globalization;
 using System.Linq;
 using System.Reflection;
+using System.Runtime.Serialization;
 using System.Text.RegularExpressions;
 using OpenRA.Graphics;
 using OpenRA.Primitives;
@@ -24,6 +25,7 @@ namespace OpenRA
 {
 	public static class FieldLoader
 	{
+		[Serializable]
 		public class MissingFieldsException : YamlException
 		{
 			public readonly string[] Missing;
@@ -42,6 +44,13 @@ namespace OpenRA
 				Header = missing.Length > 1 ? header : headerSingle ?? header;
 				Missing = missing;
 			}
+
+			public override void GetObjectData(SerializationInfo info, StreamingContext context)
+			{
+				base.GetObjectData(info, context);
+				info.AddValue("Missing", Missing);
+				info.AddValue("Header", Header);
+			}
 		}
 
 		public static Func<string, Type, string, object> InvalidValueAction = (s, t, f) =>