Print guest stack trace on invalid memory access (#1407)

* Print guest stack trace on invalid memory access

* Improve XML docs

Ryujinx.HLE/HOS/Kernel/Threading/KScheduler.cs

@@ -198,6 +198,11 @@ namespace Ryujinx.HLE.HOS.Kernel.Threading
         }
 
         public KThread GetCurrentThread()
+        {
+            return GetCurrentThreadOrNull() ?? GetDummyThread();
+        }
+
+        public KThread GetCurrentThreadOrNull()
         {
             lock (CoreContexts)
             {
@@ -210,9 +215,7 @@ namespace Ryujinx.HLE.HOS.Kernel.Threading
                 }
             }
 
-            return GetDummyThread();
-
-            throw new InvalidOperationException("Current thread is not scheduled!");
+            return null;
         }
 
         private KThread _dummyThread;