Return an error for calls into unknown interfaces or unknown objects

Ryan pointed out on IRC that we didn't do anything here. Looking at
the code, it's painfully obvious that we should be returning an error
here since a comment already says that we've exhausted all possible
options.

Signed-off-by: David Zeuthen <davidz@redhat.com>

gio/tests/gdbus-export.c

@@ -824,6 +824,18 @@ test_dispatch_thread_func (gpointer user_data)
   g_error_free (error);
   g_assert (value == NULL);
 
+  error = NULL;
+  value = g_dbus_proxy_call_sync (foo_proxy,
+                                  "org.example.FooXYZ.NonExistant",
+                                  NULL,
+                                  G_DBUS_CALL_FLAGS_NONE,
+                                  -1,
+                                  NULL,
+                                  &error);
+  g_assert_error (error, G_DBUS_ERROR, G_DBUS_ERROR_UNKNOWN_METHOD);
+  g_error_free (error);
+  g_assert (value == NULL);
+
   /* user properties */
   error = NULL;
   value = g_dbus_proxy_call_sync (foo_proxy,