#6399: protocol (based on a built-in ocaml.error extension node) to let ppx tools send located errors to be reported by the compiler (patch by Peter Zotov).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14756 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02