ovs-vsctl: Fix behavioral regression for "--if-exists del-port <bridge>".

Commit 89f3c258fe (ovs-vsctl: Improve error message for "ovs-vsctl del-port
<bridge>".) changed the behavior of
    ovs-vsctl --if-exists del-port <bridge>
from a silent no-op to a hard failure.  This commit fixes this regression.

This caused problems on XenServer, for which the Open vSwitch integration
runs commands like:
    /usr/bin/ovs-vsctl --timeout=20 \
        -- --with-iface --if-exists del-port xapi103 \
        -- --if-exists del-br xapi103

Bug #18276.
Reported-by: Michael Hu <mhu@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>