Use pg_result_error, not pg_last_error, as the latter gives false negatives.

Use pg_result_error, not pg_last_error, as the latter gives false negatives.

Specifically in the savepoint part of the code, where a failed query inside the
savepoint gives a non-false reply to pg_last_error by the time we call it,
which then leads to us doing 'RELEASE' instead of 'ROLLBACK TO', and causes
all sorts of mayhem.

The whole system needs a little cleanup, but this is an important bandaid.

Change-Id: I0223a3599816db5650f45863400e6194468d4361