Our HTTP proxy engine wrongly reports the connection was rejected

Testing http proxying using a more recent version (Server: squid/3.5.12) than the one in our network testing server (Server: squid/3.0.STABLE19) leads to a failing test in tst_qtcpsocket (tst_QTcpSocket::hostNotFound).

The reason for this seems to be that the newer squid is reporting HTTP status 503 when the host is not found, while the older version reports 404.

We check the status codes in the httpsocketengine and sets an error based on it, here:
https://code.woboq.org/qt5/qtbase/src/network/socket/qhttpsocketengine.cpp.html#675

Which then leads to a confusing message about the host rejecting our connection when it was actually not found in DNS at all.