Inconsistency in ACL error message

Hi all,

I noticed a very minor inconsistency in some ACL error messages. When
you are try and alter a role, it just says "permission denied":

postgres=> ALTER ROLE bar NOCREATEDB;
ERROR: permission denied
postgres=> ALTER ROLE bar SET search_path TO 'foo';
ERROR: permission denied

For almost all other ACL error, we include what the action was. For
example:

postgres=> CREATE ROLE r;
ERROR: permission denied to create role
postgres=> DROP ROLE postgres;
ERROR: permission denied to drop role
postgres=> CREATE DATABASE foo;
ERROR: permission denied to create database

It's not a huge deal, but it's easy enough to fix that I thought I'd
generate a patch (attached). Let me know if people think that it's
worth merging.

- Joe Koshakow