pgsql: Fix error handling in pltcl_returnnext.

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

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8c5722948e831c1862a39da2bb5d793a6f2aabab

Modified Files
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src/pl/tcl/pltcl.c | 99 ++++++++++++++++++++++++++++++++----------------------
1 file changed, 59 insertions(+), 40 deletions(-)