ovs-vsctl: Improve error message for "ovs-vsctl del-port <bridge>".

ovs-vsctl: Improve error message for "ovs-vsctl del-port <bridge>".

Previously, commands like this:
    ovs-vsctl add-br br0
    ovs-vsctl del-port br0
yielded an error message like:
    no port named br0
which is confusing.  This commit improves the error message to:
    cannot delete port br0 because it is the local port for bridge br0
    (deleting this port requires deleting the entire bridge)

Bug #17994.
Reported-by: Reid Price <reid@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>