Fix broken logic for reporting PL/Python function names in errcontext.

Fix broken logic for reporting PL/Python function names in errcontext.

plpython_error_callback() reported the name of the function associated
with the topmost PL/Python execution context.  This was not merely
wrong if there were nested PL/Python contexts, but it risked a core
dump if the topmost one is an inline code block rather than a named
function.  That will have proname = NULL, and so we were passing a NULL
pointer to snprintf("%s").  It seems that none of the PL/Python-testing
machines in the buildfarm will dump core for that, but some platforms do,
as reported by Marina Polyakova.

Investigation finds that there actually is an existing regression test
that used to prove that the behavior was wrong, though apparently no one
had noticed that it was printing the wrong function name.  It stopped
showing the problem in 9.6 when we adjusted psql to not print CONTEXT
by default for NOTICE messages.  The problem is masked (if your platform
avoids the core dump) in error cases, because PL/Python will throw away
the originally generated error info in favor of a new traceback produced
at the outer level.

Repair by using ErrorContextCallback.arg to pass the correct context to
the error callback.  Add a regression test illustrating correct behavior.

Back-patch to all supported branches, since they're all broken this way.

Discussion: https://postgr.es/m/156b989dbc6fe7c4d3223cf51da61195@postgrespro.ru