Right now, if you do a REVOKE that doesn't actually revoke anything, it works
silently. This can be a bit of a foot-gun. For example:
CREATE FUNCTION f() RETURNS int as $$ SELECT 1; $$ LANGUAGE sql;
REVOKE EXECUTE ON FUNCTION f() FROM lowpriv;
Naively, it might be expected that `lowpriv` can't execute the function, but
unless default privileges have been changed, `lowpriv` still can under the
default grant of EXECUTE to PUBLIC. Since there was no previous grant to
`lowpriv`, nothing actually changes in the ACL. This bit a client recently.
Is it worth generating a warning in this case?
