Improve error reporting for startup errors.

Since any exceptions thrown from Tasks are combined in an
AggregateException, these must be unwrapped to get the actual error
before reporting them in an error dialog.  Failure to do so results in
useless error messages like "One or more errors occurred."

Paymetheus/App.xaml.cs

@@ -24,7 +24,16 @@ public App()
 
             Application.Current.Dispatcher.UnhandledException += (sender, args) =>
             {
-                MessageBox.Show(args.Exception.Message, "Error");
+                var ex = args.Exception;
+
+                var ae = ex as AggregateException;
+                Exception inner;
+                if (ae != null && ae.TryUnwrap(out inner))
+                {
+                    ex = inner;
+                }
+
+                MessageBox.Show(ex.Message, "Error");
                 UncleanShutdown();
                 Application.Current.Shutdown(1);
             };