Fix error code for referential action RESTRICT

Fix error code for referential action RESTRICT

According to the SQL standard, if the referential action RESTRICT is
triggered, it has its own error code.  We previously didn't use that,
we just used the error code for foreign key violation.  But RESTRICT
is not necessarily an actual foreign key violation.  The foreign key
might still be satisfied in theory afterwards, but the RESTRICT
setting prevents the action even then.  So it's a separate kind of
error condition.

Discussion: https://www.postgresql.org/message-id/ea5b2777-266a-46fa-852f-6fca6ec480ad@eisentraut.org