PR#4788: wrong error message when executable file is not found for backtrace

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13966 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
follow-up to commit 13966 (PR#4788)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13967 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02