Rethink plpgsql's way of handling SPI execution during an exception block.

We don't really want to start a new SPI connection, just keep using the old
one; otherwise we have memory management problems as illustrated by
John Kennedy's bug report of today.  This requires a bit of a hack to
ensure the SPI stack state is properly restored, but then again what we
were doing before was a hack too, strictly speaking.  Add a regression
test to cover this case.