NetworkPkg/HttpBootDxe: Fix the incorrect error message output.

NetworkPkg/HttpBootDxe: Fix the incorrect error message output.

For IPv6 case, if one invalid URL returned from DHCP server, HttpBootDxe
driver could not retrieve the URL host address from DNS server. In such a
case, the error message should be printed as:
  Error: Could not retrieve the host address from DNS server.
Instead of:
  Error: Could not discover the boot information for DHCP server.
Then, we can still output as following:
  Error: Could not retrieve NBP file size from HTTP server.

Besides, currently implementation in HttpBootLoadFile will always output
error message even the HTTP process is correct.

This patch is to fix above issue.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>