Add more specific HTTP error codes to QNetworkReply::NetworkError

A few more HTTP status codes from the 4xx and 5xx series have been
added to QNetworkReply::NetworkError.

For content errors, the following codes have been added:
1. 409 - Resource Conflict
2. 410 - Resource Gone

For server related errors, the following codes have been added:
1. 500 - Internal Server Error
2. 501 - Operation Not Implemented
3. 503 - Service Unavailable

Few of the above codes are quite possible when communicating with REST
based services.

NOTE:
=====
* HTTP error status 400 is interpreted as
QNetworkReply::ProtocolInvalidOperationError.
* QNetworkReply::UnknownServerError is returned for all server related
errors (5xx) not listed above.

[ChangeLog][QtNetwork][QNetworkReply] Added more (specific) HTTP status
codes to NetworkError enum.

Task-number: QTBUG-30880
Change-Id: I9d2a133f6b3869f26710c6eb930dd8b08df31108
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>