Fix plperl and pltcl error handling per my previous proposal. SPI

Fix plperl and pltcl error handling per my previous proposal.  SPI
operations are now run as subtransactions, so that errors in them
can be reported as ordinary Perl or Tcl errors and caught by the
normal error handling convention of those languages.  Also do some
minor code cleanup in pltcl.c: extract a large chunk of duplicated
code in pltcl_SPI_execute and pltcl_SPI_execute_plan into a shared
subroutine.