Android: Report HTTP error codes as failed loads

Previously, only internal Chromium errors would get reported as
a QWebViewLoadingInfo::Failed. This change overrides the
onReceivedHttpError() method, which is called when a 400+ HTTP status
code is returned.

Since this method is called before loading of the 4XX error page
has even begun, we store the error code and message, and only
emit the signal when the load has finished. This is to keep us
consistent, and only emit a load started followed by a load
finished signal.

Pick-to: 6.12
Change-Id: Ifc48e8868b3aa873f3957fc8b1dafed069a1b091
Reviewed-by: Moss Heim <moss.heim@qt.io>