Let nm_platform_ip_route_add() and friends return an NMPlatformError
failure reason.

Also, do_add_addrroute() did not return the response from kernel.
Instead, it determined success/failure based on the presence of the
object in the cache. That is racy and does not allow to give a failure
reason from kernel.

Instead, determine success solely based on the netlink reply from
kernel. The received errno shall be authorative, there is no need
to second guess the response.

There is a problem that netlink is not a reliable protocol. In case
of receive buffer overflow, the response is lost and we don't know
whether the command succeeded (it likely did). It's unclear how to fix
that, but for now just return "unspecified" error. We probably avoid
that already by having a huge buffer size.

Also, downgrade the error message to <warn> level. <error> is really
for bugs only.