pgsql: Fix pl/tcl's handling of errors from Tcl_ListObjGetElements().

Fix pl/tcl's handling of errors from Tcl_ListObjGetElements().

In a procedure or function returning tuple, we use that function to
parse the Tcl script's result, which is supposed to be a Tcl list.
If it isn't, you get an error.  Commit 26abb50c4 incautiously
supposed that we could use throw_tcl_error() to report such an error.
That doesn't actually work, because low-level functions like
Tcl_ListObjGetElements() don't fill Tcl's errorInfo variable.
The result is either a null-pointer-dereference crash or emission
of misleading context information describing the previous Tcl error.

Back off to just reporting the interpreter's result string, and
improve throw_tcl_error()'s comment to explain when to use it.

Also, although the similar code in pltcl_trigger_handler() avoided
this mistake, it was using a fairly confusing wording of the
error message.  Improve that while we're here.

Per report from A. Kozhemyakin.  Back-patch to all supported
branches.

Erik Wienhold and Tom Lane

Discussion: https://postgr.es/m/6a2a1c40-2b2c-4a33-8b72-243c0766fcda@postgrespro.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b631d014987b9411ed0eeb7dcba9e0a8599bc3cf

Modified Files
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src/pl/tcl/expected/pltcl_call.out |  8 ++++++++
src/pl/tcl/pltcl.c                 | 11 +++++++++--
src/pl/tcl/sql/pltcl_call.sql      | 11 +++++++++++
3 files changed, 28 insertions(+), 2 deletions(-)