Fix error message for ALTER CONSTRAINT ... NOT VALID

Fix error message for ALTER CONSTRAINT ... NOT VALID

Trying to alter a constraint so that it becomes NOT VALID results in an
error that assumes the constraint is a foreign key.  This is potentially
wrong, so give a more generic error message.

While at it, give CREATE CONSTRAINT TRIGGER a better error message as
well.

Co-authored-by: jian he <[email protected]>
Co-authored-by: Fujii Masao <[email protected]>
Co-authored-by: Álvaro Herrera <[email protected]>
Co-authored-by: Amul Sul <[email protected]>
Discussion: https://postgr.es/m/CACJufxHSp2puxP=q8ZtUGL1F+heapnzqFBZy5ZNGUjUgwjBqTQ@mail.gmail.com