Review for Octave 6.

This patch is a one liner, as Olaf said, thus post it here:

# HG changeset patch
# User Olaf Till <olaf.till@uni-jena.de>
# Date 1444646177 -7200
#      Mon Oct 12 12:36:17 2015 +0200
# Node ID c59a42925f83e8d2a4e35e8bf9b55f55f7d2e366
# Parent  3339c9bdfe6a6b5cbb1c4048f36367f812fc6fac
libinterp/corefcn/cellfun.cc: error handler gets stack info.

diff -r 3339c9bdfe6a -r c59a42925f83 libinterp/corefcn/cellfun.cc
--- a/libinterp/corefcn/cellfun.cc        Sat Oct 03 07:32:50 2015 +0200
+++ b/libinterp/corefcn/cellfun.cc        Mon Oct 12 12:36:17 2015 +0200
@@ -95,6 +95,7 @@
           msg.assign ("index",
                       static_cast<double> (count
                                            + static_cast<octave_idx_type>(1)));
+          msg.assign ("stack", octave_value (last_error_stack ()));

           octave_value_list errlist = inputlist;
           errlist.prepend (msg);

In recent Octave dev, it could be applied to the following line:

https://hg.savannah.gnu.org/hgweb/octave/file/6a37ebaeeb28/libinterp/corefcn/cellfun.cc#l108

Olaf, is this patch still relevant for you/Octave?

'cellfun' can be passed an error handler. In case the user function produces an error, the error handler is called with the arguments of the user function and an additional structure with error information. The return value of the error handler is returned as an element of the return values of cellfun.

The structure with error information, passed to the error handler, contains the fields 'identifier' (error identifier), 'message' (error message), and 'index' (index of argument set for user function). However, it does not contain the error stack. This hinders finding the erroneous code in the user function.

This is not a big issue if only cellfun is used, since one can call it without an error handler to see the error stack of the user function directly. But in parallel execution, as with parcellfun or the new netcellfun, the errors of the child processes are caught and their messages can only be seen by examining the output of the error handler. It's no great problem to pass stack information to the error handler in parcellfun and netcellfun. But in some cases (chunked evaluation) these functions internally call cellfun, so for consistency, stack information should also be passed to the error handler by cellfun, so that this information is available to parcellfun and netcellfun. The stack can be passed in the field 'stack', and can be the same as the corresponding field returned by the function 'lasterror'.

Matlabs cellfun does not set the field 'stack' for the error handler, so the main problem should be to decide if we want to set it or not. The implementation itself is only one line, so I barely see the need to provide a changeset, but here it is.