HLE: misc: throw a more descriptive error when the loaded processes doesn't contain _latestPid (likely missing FW)

src/Ryujinx.Common/RyujinxException.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace Ryujinx.Common
+{
+    public class RyujinxException : Exception
+    {
+        public RyujinxException(string message) : base(message)
+        { }
+    }
+}