Autotest: Print errno in case of failure

Change-Id: I7de033f80b0e4431b7f1ffff13fc4a02e1c1a2a5
Reviewed-by: Richard J. Moore <rich@kde.org>

tests/auto/network/socket/platformsocketengine/tst_platformsocketengine.cpp

@@ -325,6 +325,7 @@ void tst_PlatformSocketEngine::broadcastTest()
                                          trollMessage.size(),
                                          QIpPacketHeader(QHostAddress::Broadcast, port));
 
+    QVERIFY2(written != -1, qt_error_string().toLocal8Bit());
     QCOMPARE((int)written, trollMessage.size());
 
     // Wait until we receive it ourselves