Re: Add schema-qualified relnames in constraint error messages.

Please find attached a POC patch, using \errverbose for the command name.  Unfortunately, I didn't see a good way to contain the change in psql only and had to change libpq, adding new interface PQresultBuildErrorMessage().  Also, what I don't like very much is that we need to keep track of the last result from last SendQuery() in psql's pset.  So in my view this is sort of a dirty hack that works nonetheless.