don't print additional error message in argument list evaluation * pt-arg-list.cc (tree_argument_list::convert_to_const_vector): Don't call error for for failed argument evaluation.

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");