add arrayfun BIST for function failing to return requested output (bu…

…g #66642)

* cellfun.cc (arrayfun): Add BIST testing for error message to be produced when
called function does not return values for requested outputs.

* cellfun.cc (cellfun): Add explanatory comment for previously added BIST.

libinterp/corefcn/cellfun.cc

@@ -1045,6 +1045,7 @@ v = cellfun (@@det, C);         # 40% faster
 %! assert (! isempty (__errmsg));
 %! clear -global __errmsg;
 
+## An error should occur if called function does not return requested outputs
 %!function [a, b] = __counterror (x)
 %!  a = x;
 %!endfunction
@@ -1674,6 +1675,13 @@ arrayfun (@@str2num, [1234],
 %! assert ([(isempty (A(1).message)), (isempty (A(2).message))],
 %!         [false, false]);
 %! assert ([A(1).index, A(2).index], [1, 2]);
+
+## An error should occur if called function does not return requested outputs
+%!function [a, b] = __counterror (x)
+%!  a = x;
+%!endfunction
+%!test <66642>
+%! fail ("[a, b] = arrayfun (@__counterror, [1, 4])");
 */
 
 static void