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 26abb50c, found more or less accidentally
by the buildfarm thanks to the tests added in 961bed02.

Report: https://postgr.es/m/30647.1483989734@sss.pgh.pa.us