Print line number correctly in COPY.

When COPY uses the multi-insert method to insert a batch of tuples into the
heap at a time, incorrect line number was printed if something went wrong in
inserting the index tuples (primary key failure, for exampl), or processing
after row triggers.

Fixes bug #8173 reported by Lloyd Albin. Backpatch to 9.2, where the multi-
insert code was added.