ovs-vsctl: Print error when add-port fails.

ovs-vsctl: Print error when add-port fails.

When the add-port command fails, vsctl reports the failure and just
suggests to check the logs for more details.

ovs-vswitchd fills the error column in the Interface table with a
description of the error, so it might be helpful to print that.

This is useful especially for dpdk devices, because the port naming
change could use a better error reporting.

I'm planning another patch to make sure that ovs-vswitch writes
appropriates information in the error column, after the dpdk port naming
changes are merged.

CC: Ben Pfaff <blp@ovn.org>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>