[ Previous patch reversed.]

Please use this patch instead of my previously submitted one.

It is just remerged against HEAD for new alter_table.out stuff.

Another reason this patch is useful for _interactive_ users: imagine a
view based on a many way join.  Imagine creating a complicated insert
rule that inserts into all the joined tables and when you insert you get
a check failure, but you need to know which actual table the constraint
was on that failed!

Christopher Kings-Lynne