No warning for a no-op REVOKE

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?