Define QNetworkReply::NetworkSessionFailed error

This is to replace the UnknownNetworkError which occurs when the
internal QNetworkSession fails to start (e.g. no usable WLAN available)

Change-Id: I2b14577c22e0acf8ff07be7e932f0dfe9ac89c33
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>

src/network/access/qnetworkreply.cpp

@@ -131,6 +131,9 @@ QNetworkReplyPrivate::QNetworkReplyPrivate()
     roaming to another access point. The request should be resubmitted
     and will be processed as soon as the connection is re-established.
 
+    \value NetworkSessionFailedError   the connection was broken due
+    to disconnection from the network or failure to start the network.
+
     \value ProxyConnectionRefusedError the connection to the proxy
     server was refused (the proxy server is not accepting requests)
 

src/network/access/qnetworkreply.h

@@ -77,6 +77,7 @@ public:
         OperationCanceledError,
         SslHandshakeFailedError,
         TemporaryNetworkFailureError,
+        NetworkSessionFailedError,
         UnknownNetworkError = 99,
 
         // proxy errors (101-199):