Handle UPnP IGDs with both WANIPConnection and WANPPPConnection services

This commit addresses the first two issues identified in #432. IGDs that
offer both WANIPConnection and WANPPPConnection services should be now
have port forwarding correctly configured for both services.

In the event of an error during discovery or configuration of any
service, the error will be returned immediately instead of attempting to
continue configuration.

Some error messages are also clarified in this commit.