PR#6069: improve ocamldoc error message

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13974 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6069: improve ocamldoc error message (bis)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13976 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02