Fix error handling in pltcl_returnnext.
We can't throw elog(ERROR) out of a Tcl command procedure; we have
to catch the error and return TCL_ERROR to the Tcl interpreter.
pltcl_returnnext failed to meet this requirement, so that errors
detected by pltcl_build_tuple_result or other functions called here
led to longjmp'ing out of the Tcl interpreter and thereby leaving it
in a bad state. Use the existing subtransaction support to prevent
that. Oversight in commit
26abb50c4, found more or less accidentally
by the buildfarm thanks to the tests added in
961bed020.
Report: https://postgr.es/m/30647.
1483989734@sss.pgh.pa.us