ErrorApplet: Implement ApplicationErrorArg (#2123)

This PR implement `ApplicationErrorArg` to the Error Applet. It's used by the guest to throw some specific error messages.
The code was done for (and merged) LDN2 build since long time ago and have been tested a bunch of times because of that! In a way to reduce the differences between LDN and master build it's fine to add it to master.

Ryujinx.HLE/HOS/Applets/Error/ApplicationErrorArg.cs

@@ -0,0 +1,13 @@
+using System.Runtime.InteropServices;
+
+namespace Ryujinx.HLE.HOS.Applets.Error
+{
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    unsafe struct ApplicationErrorArg
+    {
+        public uint       ErrorNumber;
+        public ulong      LanguageCode;
+        public fixed byte MessageText[0x800];
+        public fixed byte DetailsText[0x800];
+    }
+} 

Ryujinx.HLE/HOS/Applets/Error/ErrorApplet.cs

@@ -8,6 +8,7 @@ using Ryujinx.HLE.FileSystem;
 using Ryujinx.HLE.HOS.Services.Am.AppletAE;
 using Ryujinx.HLE.HOS.SystemState;
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
@@ -51,6 +52,12 @@ namespace Ryujinx.HLE.HOS.Applets.Error
                     {
                         ParseErrorCommonArg();
 
+                        break;
+                    }
+                case ErrorType.ApplicationErrorArg:
+                    {
+                        ParseApplicationErrorArg();
+
                         break;
                     }
                 default: throw new NotImplementedException($"ErrorApplet type {_errorCommonHeader.Type} is not implemented.");
@@ -163,6 +170,41 @@ namespace Ryujinx.HLE.HOS.Applets.Error
             }
         }
 
+        private void ParseApplicationErrorArg()
+        {
+            ApplicationErrorArg applicationErrorArg = IApplet.ReadStruct<ApplicationErrorArg>(_errorStorage);
+
+            byte[] messageTextBuffer = new byte[0x800];
+            byte[] detailsTextBuffer = new byte[0x800];
+            unsafe
+            {
+                Marshal.Copy((IntPtr)applicationErrorArg.MessageText, messageTextBuffer, 0, 0x800);
+                Marshal.Copy((IntPtr)applicationErrorArg.DetailsText, detailsTextBuffer, 0, 0x800);
+            }
+
+            string messageText = Encoding.ASCII.GetString(messageTextBuffer.TakeWhile(b => !b.Equals(0)).ToArray());
+            string detailsText = Encoding.ASCII.GetString(detailsTextBuffer.TakeWhile(b => !b.Equals(0)).ToArray());
+
+            List<string> buttons = new List<string>();
+
+            // TODO: Handle the LanguageCode to return the translated "OK" and "Details".
+
+            if (detailsText.Trim() != "")
+            {
+                buttons.Add("Details");
+            }
+
+            buttons.Add("OK");
+
+            bool showDetails = _horizon.Device.UiHandler.DisplayErrorAppletDialog($"Error Number: {applicationErrorArg.ErrorNumber}", "\n" + messageText, buttons.ToArray());
+            if (showDetails)
+            {
+                buttons.RemoveAt(0);
+
+                _horizon.Device.UiHandler.DisplayErrorAppletDialog($"Error Number: {applicationErrorArg.ErrorNumber} (Details)", "\n" + detailsText, buttons.ToArray());
+            }
+        }
+
         public ResultCode GetResult()
         {
             return ResultCode.Success;