ovs-vsctl: Log better warnings for duplicate ports and interfaces.

ovs-vsctl: Log better warnings for duplicate ports and interfaces.

The database prevents multiple ports or interfaces with a single name, but
duplicates can still occur if, for example, two bridges' "ports" columns
both point to a single Port record.  The existing warning just says in this
case that the database contains a duplicate port name.  This prompts users
to dump the Port table to look for the duplicate.  Of course there isn't
one, so then they ask me to point out the problem.

This commit improves the log message to point out the actual problem.