Supply more detailed exception details.

So we can display better errors in a designer.

src/Avalonia.DesignerSupport/Remote/RemoteDesignerEntryPoint.cs

@@ -8,6 +8,7 @@ using Avalonia.Remote.Protocol;
 using Avalonia.Remote.Protocol.Designer;
 using Avalonia.Remote.Protocol.Viewport;
 using Avalonia.Threading;
+using Portable.Xaml;
 
 namespace Avalonia.DesignerSupport.Remote
 {
@@ -204,9 +205,18 @@ namespace Avalonia.DesignerSupport.Remote
                 }
                 catch (Exception e)
                 {
+                    var xamlException = e as XamlException;
+
                     s_transport.Send(new UpdateXamlResultMessage
                     {
-                        Error = e.ToString()
+                        Error = e.ToString(),
+                        Exception = new ExceptionDetails
+                        {
+                            ExceptionType = e.GetType().FullName,
+                            Message = e.Message.ToString(),
+                            LineNumber = xamlException?.LineNumber,
+                            LinePosition = xamlException?.LinePosition,
+                        }
                     });
                 }
             }

src/Avalonia.Remote.Protocol/DesignMessages.cs

@@ -15,6 +15,7 @@ namespace Avalonia.Remote.Protocol.Designer
     {
         public string Error { get; set; }
         public string Handle { get; set; }
+        public ExceptionDetails Exception { get; set; }
     }
 
     [AvaloniaRemoteMessageGuid("854887CF-2694-4EB6-B499-7461B6FB96C7")]
@@ -23,4 +24,11 @@ namespace Avalonia.Remote.Protocol.Designer
         public string SessionId { get; set; }
     }
     
+    public class ExceptionDetails
+    {
+        public string ExceptionType { get; set; }
+        public string Message { get; set; }
+        public int? LineNumber { get; set; }
+        public int? LinePosition { get; set; }
+    }
 }