Reduce diff with trunk.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/constructors_with_record4@15492 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Reduce diff with trunk.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/constructors_with_record4@15493 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Improve error message.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/constructors_with_record4@15494 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02