Ensure exceptions flwo back correctly.

Author: jlaanstra

src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs

@@ -34,12 +34,29 @@ public override void Send(SendOrPostCallback d, object state)
             else
             {
                 var m = new ManualResetEvent(false);
+                ExceptionDispatchInfo edi = null;
+
                 m_dispatcherQueue.TryEnqueue(() =>
                 {
-                    d(state);
-                    m.Set();
+                    try
+                    {
+                        d(state);
+                    }
+                    catch (Exception ex)
+                    {
+                        edi = ExceptionDispatchInfo.Capture(ex);
+                    }
+                    finally
+                    {
+                        m.Set();
+                    }
                 });
                 m.WaitOne();
+
+                if (edi != null)
+                {
+                    edi.Throw();
+                }
             }
         }